|
Plant Disease Publications
Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona
Diseases of Urban Plants
Adapted by Mary Olsen
Plant Pathologist
Publication originally written by Richard Hine, Plant Pathologist (retired)
Introduction
Arizona is a large, square-shaped, climatically diverse state of
approximately 114,000 square miles within the north latitude lines from 32 to 37 degrees.
Geographically the state can be divided roughly into four areas; southwest, central,
southeast, and northern. These areas, in general, correspond with four climatic zones. The
zones include low desert areas (elevations below 1000 feet) that are found primarily in
Yuma, Maricopa and Pinal counties where annual precipitation is low (less than 4 inches),
frosts are rare, and high summer temperatures are typical (average daily high temperature
during June, July, August, and September are above 100°F). Intermediate
elevation areas (elevations from 1200 to 3300 feet) occur primarily in La Paz, Maricopa,
Pima, Gila, and Pinal counties. These areas receive more rainfall (10 to 13 inches
annually), occasional frosts and lower temperatures (there is approximately a 3°F
drop in temperature for every 1000 foot increase in elevation). The high desert areas with
elevations from approximately 3300 to 4500 feet, typically occurring in the counties of
Yavapai, Santa Cruz, Cochise, Gila, Graham, and Greenlee, have higher rainfall (12 to 18
inches), 74 to 100 frosts per year, and colder temperatures. The northern counties of
Arizona including Coconino, Navajo, Apache, and certain areas in Mohave, Yavapai, Gila,
and Greenlee are, in general, higher in elevation with colder winter climates. The growing
season in some of these locations averages only about 150 days a year.
A large and diverse number of plants are grown for landscaping purposes in
Arizona. They include perennial and annual ornamentals, evergreen and deciduous vines,
deciduous and evergreen shrubs and trees, conifers, palms, bamboos, turf and other ground
covers, citrus, fruit trees, and native plants.
Interestingly, in our desert environment many of the parasitic diseases in
landscape plants are caused by a limited number of plant pathogens. Most of the important
fungal plant pathogens survive in the soil and cause root, crown and wilt diseases of a
large number of unrelated plants. These include Phymatotrichopsis omnivora (Cotton
or Texas root rot), Phytophthora spp. (root and crown diseases), Pythium
spp. (seedling and root diseases), Rhizoctonia solani (seedling, root and stem
diseases), and Verticillium dahliae (wilt diseases). Other soil fungi including Fusarium
spp., Thielaviopsis basicola, and Macrophomina phaseolina are occasionally
involved as root and crown pathogens of a number of landscape plants. However, they are
insignificant when compared with the previously listed pathogens. They will not be
discussed in this publication.
Fungi that cause foliage diseases in our dry environment are rare. Two
groups, however, are important. They include the powdery mildew fungi (species of Uncinula,
Spaerotheca, Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Oidiopsis and Oidium)
and the rust fungi, including species in several genera including Phragmidium and Puccinia.
Other important plant pathogenic fungi, involved as causal agents of wood decay and canker
diseases in woody perennials, include Cytospora sp. (Cytospora canker), Hendersonula
toruloidea (sooty canker), and genera of wood rotting basidiomycetes such as
Ganoderma.
Only a few bacterial plant diseases are of significant importance to
discuss in detail. They include crown gall (Agrobacterium tumefaciens), oleander
gall (Pseudomonas syringae pv. savastanoi), fire blight (Erwinia
amylovora), bacterial necrosis of saguaro (Erwinia spp.) and wetwood or slime
flux. Foliar diseases are rare and insignificant.
Virus diseases, although of great importance in vegetable crops, citrus,
and certain ornamentals are less common and less important in urban plantings. Nematode
and mistletoe diseases, because of their widespread occurrence, will be covered. The
emphasis in this publication is to discuss primarily the major parasitic and nonparasitic
diseases that affect the most important landscape plants grown in Arizona.
Parasitic Diseases
Fungi
Phymatotrichopsis Root Rot
The most important disease of woody, dicotyledonous plants including
perennial ornamentals, perennial vines and perennial shrubs and trees in Arizona is Phymatotrichopsis
root rot (Cotton or Texas root rot) caused by a unique and widely distributed soil-borne
fungus, Phymatotrichopsis omnivora.
Figure 1. Symptoms of
Phymatotrichopsis root rot in peaches.
Note the dead trees with attached foliage.
The fungus is indigenous to and occurs in the alkaline,
low-organic matter soils of the southwestern United States and central
and northern Mexico. Of the thousands of fungi that cause disease in
plants, P. omnivora is unique in certain biological characteristics.
The fungus has one of the largest host ranges of any known fungal pathogen.
Over 2300 species of unrelated plants are susceptible to the disease.
Isolates of the fungus are non-specific in their pathogenicity. For
example, isolates that kill hundreds of deep rooted dicotyledonous,
urban plants are also pathogenic to important crop plants in Arizona
such as cotton, alfalfa, stone fruits, and grapes. Isolates from these
crop plants are in turn pathogenic to urban plants. Thus, home sites
established in old cotton or alfalfa fields, or other areas with a history
of Phymatotrichopsis root rot can become disaster areas for urban
landscapers.
The fungus has almost no method of dissemination but has the unusual
capability of surviving in soil in the absence of hosts for very long periods of time. The
disease symptoms usually appear during the summer. The fungus only infects roots of mature
plants. Seedlings are not susceptible to this disease.
Distribution of the Disease in Arizona: Heavily infested
areas of Phymatotrichopsis root rot include the flood plains
and certain tributaries of the Gila River (Safford, Duncan, Solomon,
Thatcher, Fort Thomas, Pima, Eden, Florence, Sacaton, Buckeye, Gila
Bend, Agua Caliente, Growler, Roll, Mohawk, and Dome Valley); the Santa
Cruz River (Sahuarita, Tucson, Cortaro, Avra Valley, Rillito, Marana,
Red Rock, and Eloy); the San Pedro River (Hereford, St. David, Benson,
Pomerene, Redington, Mammoth, and Winkelman); Colorado River (Parker,
Poston, Ehrenberg, Yuma, Somerton, and Gadsden) and certain locations
of the Salt River and Queen Creek. Other infested areas include Chandler,
Aguila, San Simon, Bowie, McNeal, Douglas, and Duncan.
The "mesa" (land at elevations above the influence of the
Colorado River) in Yuma County seems to be free of the disease, whereas many
"valley" sites are infested. Phymatotrichopsis root rot occurs at elevations as
high as 4,700 feet in the Elgin and Sonoita areas of Santa Cruz County, in the Nogales
area and Sierra Vista. Disease has never been detected in the Bonita area north of Willcox
to Kansas Settlement, but in the Sulphur Springs Valley, the disease is found near
Elfrida, McNeal, and Douglas.
The fungus is not uniformly distributed in local situations. Two
distribution patterns are common. In one situation the fungus may occur in many small
scattered circular areas, a "shot-gun" scenario. In other situations the
infested areas are large and few in number. This explains why the disease may occur in one
small area of a landscape throughout the property.
Figure 2. A typcial spore mat of Phymatotrichum
omnivorum. These fungal structures occur in the soil surface in shaded areas during
the summer "monsoon"season.
Symptoms: Infected plants suddenly wilt
during the summer when temperatures are high. Dead or dying foliage remains attached to
the infected plant. The roots of the infected plants are rotted and brown in color. Most
of the plants discussed in this publication when planted into areas infested with P.
omnivora show no symptoms during the first few years after planting. This is in
contrast with certain tap rooted crop plants such as cotton and alfalfa that become
infected and die the first summer after spring planting. The fungus is deep seated in the
soil, and it may simply be that roots of many susceptible plants do not grow into the
deeper, infested areas for a number of years. In grapes, for example, symptoms usually
first appear two to four years after planting. Some trees show no symptoms for five or
more years after planting. Symptom development and fungal activity is different in
plantings in low elevations in Arizona as compared to plantings above 3,600 feet. Symptoms
at low elevations consist primarily of initial stress, wilting of foliage and death of the
entire plant within a few days after initial symptoms. This death usually occurs from late
May through September. At higher elevations, plants may not wilt suddenly, but die more
slowly.
Figure 3. Strands of Phymatotropsis omnivora
growing on the surface of an infected root.
Because plants may die during the summer for reasons other than
Phymatotrichopsis root rot, it is necessary to examine root tissue for the presence of the
fungus. The only positive proof of death caused by P. omnivora is to examine the
cortical tissue of rotted, decayed roots and to identify the characteristic mycelial strands that are produced by the fungus on
the surface of cortical tissue from decayed roots.
This examination can be made by an experienced person with the use of a
hand lens. However, these strands, masses of vegetative mycelial growth that are unique to
this pathogen, can be more easily identified in the laboratory under a dissecting or
compound microscope.
Figure 4. The appearance of the strand in Fig. 3 as seen
under the compound microscope.
Phymatotrichopsis omnivora may produce fungal spore mats on the
surface of soil during the monsoon rain period.
The spore mats are occasionally seen in shaded areas at the base of
infected plants during hot, rainy periods in July and August. They are not seen at other
times. The usual size of the spore mat is 4 to 8 inches in diameter and about 1/4 inch
thick. The spore mats appear, almost mystically, overnight. The mats are initially white
in color but become brownish in color after 2 or 3 days of growth. The powdery mass of
spores produced on the surface of the mat are non-functional. They have never been
germinated, and they seem to play no role in dissemination of the pathogen.
Biology of P. omnivora: The fungus appears to occur deep in
the soil in localized patterns. P. omnivora has no effective method of
dissemination since there are no aerial or soil-borne spores that spread the fungus. The
pathogen produces two important structures, sclerotia and strands. Sclerotia enable P.
omnivora to survive in soil for many years. The sclerotia are small (up to 1/4 inch in
diameter), roundish masses of hyphae. When mature, they have a thick outer rind. Sclerotia
have been found in soil at depths up to 12 feet. Mycelial growth from germinating
sclerotia initiate root infection. The fungus then produces interwoven masses of hyphae
known as strands. The strands grow and colonize root tissue. Extensive root infection
results in wilting and plant death. Strands, utilizing infected root tissue as an energy
source, grow short distances through the soil to infect healthy roots.
Growth of strands from one host to another is probably the only available
method of spread for the fungus and explains a typical pattern of kill in home landscaping
situations where susceptible trees or shrubs are planted adjacent to each other. The
fungus may kill one plant 2 or 3 years after the initial planting. Spread occurs in later
years by strand growth from infected to healthy root tissue. Eventually, the entire group
of plants may succumb to the disease.
It should be emphasized that the fungus is only a pathogen of mature roots
of dicotyledonous plants. All monocotyledonous plants are immune to this disease. Winter
annuals escape the disease because P. omnivora is not active in cold soils.
Control: A positive identification of the pathogen on roots
is essential. In a disease situation not all roots are infected. Select roots with
discolored, rotted outer bark. The diseased roots should be about 3/4 inch thick and 6 to
9 inches in length. Microscopic examination will reveal the unique strands of the fungus
on the outer bark. Strands can be most easily identified on fresh material. However, they
can also be seen on the surface of old, dead roots.
The only real effective method of control is to plant immune or highly
resistant species in infested areas. Diocotyledonous plants grown as summer or winter
annuals normally escape infection by P. omnivora. All monocotyledonous plants,
either annuals or perennials, are immune to the disease. This includes large numbers of
ornamentals in the Amaryllidaceae, Liliaceae, Iridaceae, Palmaceae, and the large grass
family, Gramineae. Important immune plants from these plant families that are commonly
grown in Arizona include palms in the genera Washingtonia (fan palms), Phoenix
(date palms), and Arecastrum spp. (Queen palms), Agave and Yucca
spp., bamboos and many perennial ornamental grasses.
Other dicotyledonous desert plants, although not immune to the disease,
are tolerant and usually grow normally in infested areas. They include species in the
genus Cercidium, C. floridum (blue palo verde), C. microphyllum
(foothills palo verde), C. praecox (Sonoran palo verde); the genus Prosopis, P.
velutina (honey mesquite), P. chilensis (Chilean mesquite); Simmondsia
chinensis (jojoba); Parkinsonia aculeata (Parkinsonia), Condalia lycioides
(gray thorn) and Chilopsis linearus (desert willow). Other plants include Amorpha
fruiticosa (false indigo), Celtus spp. (hackberry), A. greggi (catclaw),
Larrea tridentata (Creosote), Fouquieria splendens (ocotillo).
There are probably many other native plants in this category. As interest
increases in the use of drought and heat tolerant native plants for landscaping purposes,
our list of resistant plants may increase. We do know that some commonly planted
non-native dicotyledonous trees and shrubs such as mulberry, Aleppo pine, and citrus are
rarely affected by the disease. The list of susceptible plants is more reliable than
plants in the resistant list. This, of course, is due to the fact that plants that succumb
to the disease can be easily catalogued whereas plants that appear resistant may appear so
simply because they have not been exposed to the pathogen. Susceptible plants grown in
Arizona include: Prunus spp. including almonds, apricots, cherry, sweet cherry,
sour cherry, nectarines, plums, peaches; Fraxinus spp. including Arizona ash (F.
velutina); cottonwood, (Populus deltoides), Ulmus spp. including chinese
elm (U. parvifolia) and siberian ulm (U. pumula); Ficus spp. (figs);
honey locust (Gleditsia spp.); magnolia (Magnolia grandiflora); pear (Pyrus
communis); pecan (Hicoria pecan); pistachio (Pistacia spp.); pomegranate
(Punica granatum); sycamore, american and london plane (Platanus occidentalis
and P. acerifolia); chinaberry tree (Melia azedarach); willow, weeping and
golden (Salix babylonica and S. alba), bottle tree (Brachychiton
populneum), silk oak (Grevillea robusta); pepper tree, Brazilian and California
(Schinus terebinthifolia and S. molle); Japanese privet (Ligustrum
japonicum); African sumac (Rhus lancea), xylosma (Xylosma congestum),
oleander (Nerium oleander), carob (Ceratonia siliqua) and Cassia.
Other genera with susceptible species include: Vitis spp. (grape), Ilex spp.
(holly), Cotoneaster spp. (cotoneaster), Malus sylvestris (apple), Juglans
spp. (walnut), Buddleia spp. (butterflybush), Syringa spp. (lilac), Pittosporum
spp. (Pittosporum), Wistaria spp. (wisteria), Rosa spp. (roses), Rubus spp.
(berries), and Spiraea spp. (spirea).
Phytophora Diseases
The genus Phytophthora contains approximately 35 species of highly
pathogenic and destructive plant pathogens. All species are pathogenic. They attack
hundreds of species of higher plants throughout the world causing unusually diverse types
of plant diseases including leaf and shoot blights, flower, fruit and bud rots, crown
rots, stem and trunk cankers of woody and herbaceous plants, seedling damping-off,
necrosis of feeder roots and collar and root rots.
In Arizona, because of our desert climate, fungus diseases that affect
above ground plant parts (foliage, flowers, fruit, etc.) are rare. Thus, infections by
species of Phytophthora most commonly occur below ground or at the soil line.
Typically, these diseases include seedling damping-off, necrosis of feeder roots, rots of
mature roots, and crown and collar rots of woody and herbaceous plants. The identification
of a Phytophthora as the cause of a root or crown disease is difficult because
below ground infections usually cause non-specific above ground symptoms. Also, the fungus
is microscopic and difficult to isolate and identify from infected below ground tissues.
Most studies in Arizona concerning diseases caused by Phytophthora
species have involved commercial crops and not plants used in urban situations. These
diseases include root rot of alfalfa (P. megasperma), safflower (P. drechsleri),
citrus (P. parasitica and P. citrophthora), root and crown rot of apple (P.
cactorum and P. cambivora), crown, stem, and fruit rot of pepper (P. capsici),
fruit rot of pumpkin (P. capsici) and boll rot of cotton (P. capsici). The
last three diseases occur only during the summer rainy season at elevations above 4000
feet in southeast Arizona.
Several of these species have very wide host ranges. For example, P.
parasitica, which is the major pathogen causing root disease in citrus grown in
Arizona, causes root and crown diseases of the following commonly grown landscape plants in
Arizona; Grevillea robusta (silk oak), Leucophyllum frutescens (Texas
ranger), Hibiscus spp., Petunia spp., Rosmarinus officinalis
(rosemary), Simmondsia chinensis (jojoba), Verbena spp., and Vinca
spp. (periwinkle).
Other diseases in Arizona where a species of Phytophthora
has been isolated include root and crown rot disease of Matthiola
spp. (stock), Antirrhinum majus (snapdragon), Aster spp.,
Lilium spp. (lily), Paeonia spp. (peony), foliage plants
including Peperomia spp., Hedera spp. (ivy), and Dieffenbachia
spp. Other Phytophthora root and crown diseases occur in Juglans
spp. (walnut), Pinus spp. (pines), Prunus spp. (peach,
apricot, cherry), Pyrus communis (pear), Quercus spp.
(oaks), Salix spp. (willows), Ilex spp. (holly), Juniperus
spp. (junipers), Cerotonia siliqua (carob tree), Bougainvillae
spp., and Hicoria pecan (pecan). No foliar diseases caused by
Phytophthora spp. have been described from landscape plants in
Arizona. Phytophthora bud rot of palms, caused by P. palmivora,
is a serious disease in the wet tropics. During our monsoon, rainy season
this disease has been occasionally seen in Arizona.
Symptoms: A number of Phytophthora spp., including P.
parasitica and P cactorum infect roots and crown tissue of host plants ranging
from herbaceous annuals to woody perennials. Symptoms, however, resulting from these
infections are basically similar. As an example, symptoms of two diseases, Phytophthora
crown and root rot of petunia (an herbaceous annual) caused by P. parasitica and
collar rot of apple (a deciduous woody perennial) caused by P. cactorum have much
in common. Symptoms, although obviously developing in different time frames, are similar
and consist of overall plant stress, stunting, change in leaf color, wilting, and eventual
plant death.
Figure 5. A trunk canker caused by Phytophthora
parasitica in citrus. Note the discolored lesion in the bark. The
fungus can be isolated from the lesion.
Phytophthora parasitica infects the lower stem of petunia at or
slightly below the soil line. The fungus grows into and kills stem tissue. Infected tissue
rots and becomes dark in color. As the disease progresses, the entire plant wilts and dies.
The disease is most common in seedlings transplanted during hot weather in September and
October. Optimum temperature for fungal activity is about 85°F. Crown rot of
apple has many similarities to this disease. The main pathogen, P. cactorum,
infects the trunk at or below the soil line. Initially, during the summer, leaves of
infected trees may appear light green to yellow in color. Fruit may be small and color
prematurely. The tree may be stunted. The optimum time to survey for collar rot of apples
is during September, October, and November. During this period, infected trees appear
stressed with stunted and dead terminal growth. Leaves are reddish-yellow in appearance. A
dark brown canker at or below the soil line is a characteristic symptom.
If no cankers are
found on trees, then factors other that Phytophthora are involved. Active cankers
are moist in appearance. An exudate may occur at the margins of the canker. Infections are
most common and canker development most active during cool-cold weather. This is because
the optimum temperature for growth of P. cactorum is approximately 50 to 60°F.
Disease development is inhibited at soil temperatures above 80°F.
Figure 6. Crown rot of apple caused by Phytophthora
capsici. Note that the canker is below the soil line.
The symptoms described for the petunia and apple diseases are essentially
identical for the Phytophthora diseases that affect the other herbaceous and woody
plants described in the previous section.
Biology: The basic biology of Phytophthora spp. that
occur in Arizona is very similar. All species survive and live indefinitely in soil.
Thick-walled, microscopic survival structures, oospores and chlamydospores, are produced
in infected host tissue. There is variation between species as to the presence or absence
of these spores and also to their size and shape. The most reliable method of identifying Phytophthora
spp. is the finding of the sporangial stage. They are commonly produced on infected tissue
and can be seen on microscopic examination of isolations made on specific media. Oospores
and chlamydospores germinate under saturated soil conditions to produce characteristic,
microscopic, lemon shaped sporangia. Each sporangium liberates swimming spores called
zoospores. They swim through water and contact roots or the lower trunk tissue of potential
hosts where they germinate and infect these tissues. The fungus grows into host tissue,
girdling the infected root or trunk and eventually killing the tree. As the host dies the
fungus produces the survival structures in decaying tissue. If the tree is removed these
spores are returned via rotted host tissue to the soil where they enable the fungus to
survive in the absence of a host. Few studies have been made on the natural occurrence of Phytophthora
spp. in Arizona soils. However, studies in Arizona support the thesis that Phytophthora
pathogens of urban plants are commonly introduced into Arizona on imported nursery stock.
Control Herbaceous Plants: The control of
Phytophthora diseases in bedding plants and other herbaceous plants starts with the
purchase of disease-free plants. For the homeowner this is sometimes difficult. Young
seedlings, petunias, for example, may be infected but show no symptoms. The shock of
transplanting, and overwatering coupled with high temperature triggers disease
development. All Phytophthora diseases occur primarily in overwatered, heavy,
poorly drained soils. Any technique that improves soil percolation is helpful in disease
control. There are a number of fungicides available as drenches that are highly active
against Phytophthora spp.
Woody plants: Avoid planting into heavy, poorly
drained sites. Break open any caliche area that prevents water percolation. Avoid
overwatering. Once collar rot has been identified there are few options available for
control. A systemic fungicide is applied as a drench around the trunk of infected trees.
Severely infected trees cannot be saved. The fungicide is effective in controlling the
spread of the disease and also for treatment of early infections. Certain citrus are
susceptible to Phytophthora foot and root rot and others are highly resistant. For
example, susceptible varieties include lemons (Citrus limon), sweet orange (C.
sinensis), limes (C. aurantifolia), grapefruit (C. paradisi), and Troyer
and Carrizo citranges. Resistant citrus commonly planted in Arizona is sour orange (C.
aurantium).
Pythium Diseases
It is appropriate, for a number of reasons, to compare the genus Pythium
with the previously described genus, Phytophthora. First, although the two genera
are closely related, they differ in certain biological characteristics. For example, all
described species in the genus Phytophthora are plant pathogens. The isolation of
or even observation of a species of Phytophthora in diseased plant tissue is
sufficient evidence implicating the fungus as the cause of the disease.
Pythium spp., however are ubiquitous in soil and aquatic
environments. Most of the 100 species described infect seeds, roots,
or aerial parts of a wide range of plants. Some species, however, do
not cause disease and are saprophytic on decaying plant debris. Species
of Phytophthora often invade woody, mature plant tissue. Pathogenic
Pythium spp. invade and cause disease primarily in young meristematic,
herbaceous tissue. Species in both genera survive indefinitely in the
soil and, under desert conditions, cause diseases primarily of below
ground tissues. Phytophthora spp. have limited soil saprophytic
capabilities. They are slow growing and isolated with difficulty from
infected tissue. Specialized, specific media are necessary for isolation.
Pythium spp., in contrast, are easily isolated from infected
host tissue on water agar or other simple media.
There is also, in general, less host specificity in Pythium than in
Phytophthora. The two most important Pythium spp. in Arizona, P.
aphanidermatum and P. ultimum are aggressive, rapidly growing soil saprophytes.
Figure 8 (left) and Figure 9 (right). These two postemergence seedling diseases (citrus, Figure 8, and beans, Figure 9) could be caused by infection with a number of soil occurring fungi including species in the genera Pythium, Phytophthora or Rhizoctonia.
Figure 10. Pythium blight in turf grass. Note the
irregular, dead areas where the fungus is active.
They readily colonize soil organic matter. Infection of below ground plant tissue occurs, as with
species of Phytophthora, under saturated soil conditions. The
two species differ primarily in their response to soil temperature.
Pythium ultimum is pathogenic primarily in soils that are near
or below 60°F whereas P. aphanidermatum has optimum
temperature requirements for pathogenicity above 85°F.
Symptoms: Pythium ultimum and P.
aphanidermatum are primarily involved in seedling diseases of bedding and ornamental
plants. They are both indigenous in Arizona soils. Both species are known pathogens of a
number of agricultural crops grown in Arizona including several vegetable and field crops.
In landscape plants, however, these two species are important as causal organisms in seed
decay, pre and post emergence damping off of seedlings and root rot of immature or
herbaceous plants. Seeds or seedlings rot before or after their emergence from the soil.
Post emergence damping off can occur several weeks after emergence. Seedlings become less
susceptible with increasing age.
Pythium root rot is also a common disease in many foliage plants including
Philodendron, Peperomia, Scindapsus, and Syngonium. A
wide range of young greenhouse, bedding, and garden and landscape plants
are susceptible to Pythium root rot. For diagnosis, affected
plants should be carefully removed from the soil and roots washed in
water. Roots and lower stems are often brown or black. The stem slightly
above or below the soil line may be smaller in diameter at the infection
site than healthy stem tissue. Damping off in flats of bedding plants
often starts in the corner or center of the flat. The fungus, after
introduction into pasteurized soil, spreads rapidly from diseased to
healthy seedlings. Temperatures that adversely affect seed germination
and seedling growth increase disease incidence. Wet and poorly drained
soils also are factors in increased incidence of disease.
Figure 7. Root lesions caused by
Rhizoctonia solani (left) and Pythium spp. (right).
Note the distinct, dunken, reddish lesion caused by Rhizoctonia
in comparison with the overall, blackened root infected with Pythium.
In greenhouse situations, P. aphanidermatum is a serious problem
only when air and soil temperatures are above approximately 85°F. Pythium
ultimum, is not pathogenic at these temperatures and becomes active in cold (below 65°F),
wet soils.
Biology: Both species produce oospores. Pythium
aphanidermatum produces sporangia and zoospores. Pythium ultimum produces
sporangia. These sporangia, however, germinate with the production of mycelia and no
zoospores are produced. They both have very wide host ranges and both are non-specific in
their host specificity. In other words, an isolate of P. aphanidermatum that causes
seedling disease in tomatoes will also cause disease in petunia seedlings. Because of the
production of oospores both species survive indefinitely in soil.
Control: Garden crops that are seeded directly in the soil
are best protected against seed and seedling disease by treating seeds with a fungicide.
Most commercial seed bought by landscapers has been treated with fungicides. Any factor
that aids in quick germination and rapid seedling growth reduces the chance of Pythium
seedling diseases. These factors include proper planting period, careful preparation of
the seed bed to increase drainage, no overwatering, use of high quality seed, and proper
fertilization.
Rhizoctonia Diseases
Although there are several species of Rhizoctonia described as
plant pathogens, R. solani, is the most important species in the genus.
Rhizoctonia solani, a soil borne fungus, combines aggressive
saprophytic activity with almost unlimited pathogenic capabilities. The fungus is a
worldwide pathogen of hundreds of unrelated plants grown in climates as diverse as our
irrigated desert to the humid tropics. In Arizona, R. solani is an important
pathogen of many of our agricultural crops including cotton, potatoes, lettuce and many
other vegetables and field crops. The fungus causes primarily seed
decay, pre- and postemergence damping off of seedlings, bulb rots, root rots, crown
blights, neck rots and lower stem rots. Almost any plant tissue in contact with the soil
is susceptible to infection. Diseases of aerial plant tissue, including upper stems,
leaves, flowers, and fruit, caused by R. solani, are common in humid areas but are
rare in our desert environment. Rhizoctonia solani does not invade and cause
disease, under our conditions, in woody plant tissue. Established woody plants including
trees, shrubs and vines are not affected by Rhizoctonia. The fungus, as with Pythium
spp., is a pathogen primarily of young, succulent, immature tissue.
Symptoms: During seedling germination and emergence, the fungus
causes partial or complete girdling of emerged seedlings at or near the soil surface. The
lesions on lower stems are sunken and dark in color. When the soil is moist the fungus
grows as weblike mycelium on the soil surface and in the lesions. Mycelial growth is
easily visible with a hand lens. Most infections of seedlings and cuttings occur at or
just below the soil surface. Infected seedlings wilt and die. Cuttings never root, and they
eventually collapse. In plants like dichondra (Dicondra repens), the fungus invades
and causes brown lesions on the lower stems. The lesions are sunken and dark in color.
Mycelial growth is evident in the infected areas. Similar infections occur in periwinkle (Vinca
major) and foliage plants such as Peperomia. Rhizoctonia solani may
blight a large stand of turfgrass within a 24 to 48 hour period. The fungus invades young
leaf tissue and grows rapidly under warm temperatures (75 to 85°F) and the high
humidities that can exist with excessive watering and high mowing of the turf. With
cuttings of carnation, a soft, wet rot occurs at the point of callus formation. Infected
plants wilt rapidly and die.
Hosts: Rhizoctonia solani has one of the
largest host ranges of any fungal plant pathogen. The fungus causes disease in species in
530 genera of higher plants, including both dicots and monocots, from Abelia to Zoysia
grass. The fungus causes seedling diseases and lower stem and root diseases of a large
number of plants used in urban plantings in Arizona including aloe (Aloe spp.),
ageratum (Ageratum spp.), aster (Aster spp.), coleus (Coleus spp.),
chrysanthemum (Chrysanthemum spp.), carnation (Dianthus spp.), dichondra (Dichondra
spp.), geranium (Geranium and Pelargonium spp.), iris (Iris spp.),
lantana (Lantana spp.), lilies (Lilium spp.), marigold (Tagetes
spp.), narcissus (Narcissus spp.), opuntia (Opuntia spp.), periwinkle (Vinca
spp.), rose (Rosa spp.), snapdragon (Antirrhinum spp.), sweet alyssum (Lobularia
maritima), sweet peas (Lathyrus odoratus), tulips (Tulipa spp.), violets
(Viola spp.), and zinnia (Zinnia spp.). Many house plants such as African
violets (Saintpaulia spp.), Dieffenbachia, Syngonium, Peperomia, and
others are affected with basal stem rots caused by R. solani. Several lawn grasses
are susceptible to damage caused by R. solani. They include bent grass (Agrostis
spp.), bermuda grass (Cynodon dactylon), fescue (Festuca spp.), ryegrass (Lolium
spp.), bluegrass (Poa spp.), zoysia (Zoysia spp.), and St. Augustine (Stenotaphrum
secundatum).
Biology: R. solani commonly lives in the soil (soil
borne) as a saprophyte. Quantitative estimations of soil populations of R. solani
are based on sieving organic debris from the soil and determining the percent of debris
that has been colonized by R. solani. This technique is based on the biological
fact that R. solani aggressively grows as mycelium in soil and colonizes organic
debris. The fungus survives in the debris by the production of thick walled mycelium
(resting mycelium) or by the production of hard resting structures called sclerotia. Plant
exudates stimulate mycelial growth from the resting mycelium or from the sclerotia. The
mycelium then invades host tissue and causes lesions and girdling typical of the disease.
This simple life cycle is complicated by the production, under certain conditions, of the
perfect stage of the fungus which has been named Thanatephorus cucumeris. Basidia,
specialized structures produced on host tissue under humid conditions, produce air-borne
spores called basidiospores. The basidia and basidiospores are minute and are only
observable under a microscope. In our desert conditions, this aerial, infective stage is
rarely seen and is not important in the epidemiology of the disease. In humid areas of the
world the basidiospores may initiate infection of leaves, flowers and fruit. All isolates
of R. solani are identified by their typical mycelial growth characteristics. Also,
all isolates have many nuclei in each cell of the mycelium and are considered to be the
imperfect state of Thanatephorus cucumeris. The species exists in several
non-interbreeding populations that vary in their host range. No studies have been made in
Arizona on the host ranges of isolates recovered from urban landscape plants.
Rhizoctonia solani is a rapidly growing fungus that is easily
isolated from infected tissue on simple media such as water agar.
Control: Urban plants are bought primarily from nurseries
and other commercial suppliers. Pathogens may be introduced on planting material purchased
from these facilities. The first step for control is the selection of vigorous, healthy
plants at the time of purchase. Nurseries have a strict management program to produce
healthy plants. The programs are based on fungicide treatment of seed (Rhizoctonia
can be seed-borne), heat treatment of soil (30 minutes at a minimum of 140°F),
fumigation of soil and sanitation of containers, benches, and tools.
Verticillium and Fusarium Wilts
There are two unrelated soilborne pathogenic fungi that infect host plants
through root tissue and become systemic in vascular (xylem) tissue. The plugging, enzyme
and toxin production by growth of these fungi in water conducting tissue causes typical
symptoms and often death of infected plants. These two fungi, both worldwide pathogens of
hundreds of unrelated plants, are Verticillium dahliae and Fusarium oxysporum.
Both fungi are persistent in the soil for many years in the absence of any
host. Fusarium oxysporum consists of many specialized forms which are specific to
one host. Verticillium dahliae consists of several groups, some of which parasitize
many hosts, and others which seem to be very host selective.
Both fungi invade and infect only the vascular tissue of host plants.
Overlapping non-specific symptoms occur in plants infected by both pathogens. In general
there is yellowing and drying of leaves, overall plant stress, wilting and death of
branches, and varying degrees of vascular discoloration. The only method to prove the
cause of the disease is to isolate and identify the pathogen from infected vascular
tissue.
Hosts of Verticillium dahliae include over 120 species in unrelated
plant families including many woody and herbaceous ornamentals. Forms of Fusarium
oxysporum cause disease in many food, fiber and ornamental plants. The species is one
of the most important worldwide plant pathogens. Isolates are host specific. For example,
an isolate from carnations is only pathogenic to certain cultivars in that species.
Interestingly, although both fungi occur naturally in Arizona soils only V.
dahliae is an important plant pathogen. The lack of importance of Fusarium wilts
(caused by biotypes of F. oxysporum) in Arizona is thought to be due to our
alkaline soils which are not favorable for activity of the fungus. Verticillium dahliae,
however, is an important plant pathogen of several of our important agricultural crops,
including upland cotton (Gossypium hirsutum), peppers (Capsicum spp.),
tomato (Lycopersicon esculentum), eggplant (Solanum melongena), okra (Hibiscus
esculentus), and safflower (Carthamus tinctorius). Susceptible woody-landscape
plants include olive (Olea europaea), pistachio (Pistacia chinensis, P. vera),
ash (Fraxinus spp.), maple (Acer spp.), pecan (Carya illinoensis),
carob (Ceratonia siliqua), almond, apricot, cherry, peach (Prunus spp.),
California pepper tree (Schinus molle), elm (Ulmus spp.), rose (Rosa
spp.), and privet (Ligustrum spp.). Ornamental plants susceptible to Verticillium
wilt among others include; asters (Aster spp.), chrysanthemums (Chrysanthemum
spp.), dahlia (Dahlia spp.), geranium and pelargonium (Pelargonium spp.),
petunia (Petunia spp.), phlox (Phlox spp.), stock (Matthiola spp.),
snaps (Antirrhinum majus), and sweet peas (Lathyrus odoratus). The list of
resistant or immune plants is very large and includes all cacti; all monocots such as
grasses, palms, iris and lilies; and all gymnosperms such as junipers, pines, and cypress.
Some other commonly planted shrubs and trees in Arizona that
are resistant or immune to Verticillium wilt include: apples,
citrus, eucalyptus, Pyracantha, mulberry, oak, oleander, sycamore, poplar,
walnuts, and willows.
Although Verticillium wilt is common in several agricultural crops, the
disease is less important in landscape plants. Verticillium wilt diseases of urban plants
in Arizona seem to most commonly occur in plantings made in old agricultural land with a
history of Verticillium wilt. This is because the fungus combines a wide host range with
long term survival in the soil.
Biology of V. dahliae: One has to have great respect for the
biological characteristics that make V. dahliae one of the most widespread and
destructive soilborne fungal plant pathogens. These characteristics are: rapid host
colonization via xylem (water conducting elements), production of large numbers of
reproductive spores (condidia and microsclerotia), effective asexual mechanisms of
variation, long term survival of microsclerotia in soil (up to 14 years in non-cropped
field soils), wide host range, and effective dissemination mechanisms (microsclerotia are
carried in air, dust, water, soil, plant residues, and in leaves, stems, and seed).
Another factor contributing to the success of V. dahliae
as a plant pathogen in arid desert areas is the lack of dependence of
the fungus on rain and high humidity for infection and host colonization
(the fungus is internal in the host). Also, there are many plants (especially
weeds) that, although susceptible to Verticillium wilt, show
no symptoms when they are infected (symptomless carriers).
The fungus has a simple life cycle and produces only two types of
reproductive structures: conidia and microsclerotia. The microscopic conidia are short
lived but are factors in rapid internal host colonization. The microsclerotia are black,
heavily walled, microscopic aggregates of fungal hyphae. The fungus can be identified
under the microscope by noting the whorled arrangement of conidiophore branches and the
typical clumped masses of the black microslerotia.
Microsclerotia are produced in large numbers in infected host tissue
during cool weather. They are returned to the soil on death of the host plant. They
germinate in the soil to initiate root infection.
Symptoms and Biology of Verticillium Wilt of Olive:
Because of the importance of this disease in Arizona, it will be used
as an example to discuss symptoms and biology of a typical Verticillium
disease.
Verticillium initially invades the root system of olives when soil
temperatures are cool. The fungus is relatively inactive at temperatures above 85°F.
After penetrating the roots, the fungus grows and moves through the plant in the
water-conducting (vascular) tissues and eventually invades branches and twigs. This
systemic invasion usually occurs from February to June. With the onset of high summer
temperatures the fungus is inactivated. By then, unfortunately, the damage has been done
and the trees begin to exhibit symptoms. The presence of the fungus in the vascular system
interrupts and reduces the water movement from the roots to the leaves. Wilt symptoms are
attributable to impeded internal water flow.
Symptoms usually first appear in the spring near flowering time. Newer
leaves roll inward and lose their deep-green, waxy luster and become dull gray and brown.
Leaf-drop and twig die-back may follow, depending upon the severity of the infection and
the effect of the environment on the water demand of leaves and fruit. Flower clusters on
affected branches may die and remain attached. Individual branches, portions of trees or
the entire crown may die in one season. Tree death, however, rarely occurs. New growth may
develop from unaffected portions of the trees. Suckering from the crown may be prolific.
In Arizona, a slight vascular discoloration of affected twigs and branches
may develop. This discoloration can be observed by removing the outside layer of bark from
affected branches and exposing the vascular tissue.
When cool weather returns, the fungus again becomes active. In this way
branches can be re-invaded each year during the spring season.
Control: Control of Verticillium wilt diseases is
based primarily on the use of resistant cultivars or immune species. Many resistant
cultivars of ornamental plants are available from nurseries and seed companies.
Powdery Mildew
Powdery mildew diseases occur on hundreds of plants grown in urban
landscapes. The diseases primarily affect leaves, green stems, flower buds, and immature
fruit of a wide range of plants including ornamental flowers, broad leaved shade trees,
cereals, weeds, vegetables, fruit trees, and native plants. All of the powdery mildew
fungi are obligate parasites and reproduce on living host tissue. They do not grow
saprophytically. This is in contrast to the previously described fungi which combine
saprophytic and parasitic stage in their life cycles. The powdery mildew fungi grow
primarily on leaf surfaces and aerial plant parts. They are not systemic in plants, and
they do not infect roots. The mycelium is superficial on host tissue in most genera, but
in some such as Oidiopsis, the mycelium is internal. Chains of asexual colorless
conidia are produced from conidiophores that grow from the mycelium. The name powdery
mildew accurately describes the appearance of the fungus on host tissue. Nutrients are
obtained from host tissue by means of microscopic, rootlike organs called haustoria. These
specialized fungal structures are produced only in epidermal cells.
Symptoms: Plants infected with powdery mildew fungi
typically have a whitish, powdery covering over infected leaves, stems,
and other aerial plant parts. Infected leaves are often curled or twisted.
Severe leaf infection may cause leaf yellowing, reduced leaf size and
defoliation. These symptoms are general. They are influenced by climate,
host location (shade effects), host and the causal species of mildew.
The only real method of identification of a powdery mildew disease is
to identify the host and the fungus. The fungus is easily identified
under the microscope by the characteristic growth habit of the conidial
(asexual stage) and by the architecture of the sexual fruiting body
(cleistothecium). The conidial stage is always present. The sexual stage
of the fungus may or may not be present.
Figure 11. Typical symptoms of powdery mildew on Euonymous.
Note the superificial whitish blotches of mycelium on the upper leaf surface.
Biology: Under mild winter conditions in the desert areas of
Arizona, the powdery mildew fungi occur primarily in their asexual, conidial stage. The
sexual stage is not important for survival. Thus, the life cycle is very simple. Conidia
are wind carried to host tissue where they germinate and infect host tissue. The conidia
germinate on dry leaf surfaces even when atmospheric humidity is low. Spore germination
and penetration of tissue usually occurs in 6 hours or less. Many species can produce a
new generation of spores in 4 to 6 days under favorable conditions.
Some species of the powdery mildew fungi have very large host ranges. One
of the most common species in Arizona is Erysiphe cichoracearum. This fungus causes
powdery mildew diseases in species in over 160 genera of unrelated plants from Ageratum
and Antirrhinum (snapdragons) to Verbena and Zinnia. Another species,
E. polygoni attacks species in many genera of plants from Acacia to Vicia
and Vigna. Erysiphe graminis causes powdery mildew of grasses.
Some powdery mildews of deciduous plants overwinter as dormant mycelium in
leaf buds and in bark. In the spring the fungus becomes active and the conidial stage is
produced. Powdery mildews that produce a sexual stage survive as cleistothecia. Sexual
spores called ascospores are produced in the cleistothecia and are the primary source of
infection during plant growth in the spring.
Control: Powdery mildews are controlled by the use of
resistant varieties and also by a number of fungicides, including different formulations
of sulfur. These chemicals are used as protective sprays, and it is very important to
apply them at the first sign of disease since powdery mildew is extremely difficult to
control once established.
Rust Diseases
Rusts are an amazing group of fungi belonging to a single order of
basidiomycetes, the Uredinales. Throughout history these fungi have been responsible for
catastrophic diseases of many of the world's most important plants including grain crops
such as barley, corn, oats, rye, sorghum, and wheat; vegetables such as asparagus and
beans; field crops such as cotton, safflower, soybeans, and sunflower; trees such as
apple, peaches, pines, firs, junipers, and oaks; and many ornamentals. Many plants found
in urban environments in Arizona including asters, carnations, chrysanthemums, dichondra,
geraniums, hollyhock, iris, lily, rose, and snapdragon are affected with rust diseases.
Some common woody plants affected with rust diseases in Arizona include firs (Abies
spp.), Acacia spp., cypress (Cupressus spp.), ocotillo (Fouquieria
splendens), fairy duster (Calliandra spp.), junipers (Juniperus spp.),
pines (Pinus spp.), aspen (Populus spp.), pear (Pyrus communis), and
oaks (Quercus spp.).
Symptoms: Symptoms of rust diseases are almost as diverse as the
fungi that cause the diseases. Some rust fungi have unbelievable, complex life cycles.
These life cycles may involve totally unrelated host plants and may
include up to four different spore producing stages and five functionally different kinds
of spores. Some species complete their entire life cycle on one host (autoecious) whereas
other species require two hosts (heteroecious).
In order to understand symptoms caused by infections by the rust fungi, it
is necessary to understand the various spore stages of the fungi. Infection by each spore
stage causes somewhat different symptoms. Spore stages useful in identification of the
disease include aecia, uredia, and telia. The aecia are cup and blister like in
appearance. Light yellow to orange wind dispersed aeciospores are produced in the aecia.
Uredia are also blisterlike pustules that produce masses of orange to rust-colored
uredospores. Telia are structures that produce masses of teliospores. The telia follow
uredia on the same host. The teliospores are usually black in color. Aecia, uredia, and
telia can be easily observed on infected host tissue with the use of a hand lens. The
spores, produced on each of these structures, however, are microscopic.
Identification of rusts may be complicated because many rusts omit one or
more of the above stages. Also, aecia and uredia are somewhat similar in appearance.
Symptoms of many rust diseases consist primarily of these small pustules on the underside
of leaves.
Although the rust fungi most frequently attack leaves they also infect
flowers, fruit, and stems and branches and trunks of woody flowering plants and conifers.
Most rust infections are localized but in some diseases the fungi become locally systemic
in host tissue and cause galls, blisters, and cankers in woody tissue.
Biology: The rust fungi, like the powdery mildew fungi, are
all obligate parasites. Thus, it is impossible to isolate and grow these fungi in the
laboratory. They complete their life cycles only on living host plants.
Variation in the life cycles of rust fungi can be dramatically illustrated
by comparing three common rust diseases in Arizona: snapdragon rust (Puccinia
antirrhini), hollyhock rust (P. malvacearum), and limb rust of Ponderosa pine (Cronartium
arizonicum). The uredial and the telial stage occur on snapdragons and only the telial
stage on hollyhock, respectively. In contrast to these two simple cycles is the complex
cycle of the limb rust pathogen. Aeciospores, produced from aecia on twigs of pine trees
infect a totally unrelated host, paintbrush (Castilleja integra). Uredia and telia
are produced on paintbrush. The urediospores infect only paintbrush. Teliospores, produced
from telia on paintbrush, germinate and produce basidiospores that infect twigs of pine in
the fall.
Snapdragon rust and hollyhock rust are examples of types of short cycle
rusts and limb rust is an example of a long cycle rust. Almost any combination of spore
stages is found in nature. The extraordinary biology of these fungi has fascinated plant
pathologists throughout history. From an epidemiological consideration it should be noted
that aeciospores in heteroecious rusts always infect the alternate host. Aeciospores may
be aerially disseminated for hundreds of miles and remain viable. Urediospores normally
infect the same host on which they develop. They may or may not be aerially disseminated
for long distances. Basidiospores, produced from overwintering teliospores, are thin
walled and delicate and are not adapted for long range spread. In heteroecious rusts
(alternate hosts necessary for completion of life cycle) the basidiospores always infect
the alternate host.
Control: The first step in controlling a rust disease, as
with all plant diseases, is the identification of the causal fungus. The rust fungi are
identified primarily by the anatomy of the aeciospores, urediospores and the teliospores.
These spores are microscopic and must be examined with a compound microscope. Also, it is
necessary to know the host species. Some hosts are infected by more than one species of
rust fungi. Other hosts have only a single rust pathogen. Symptoms of each disease are
different and very important in disease identification.
Control of rust disease is as complicated as the biology of the rust
fungi. Each disease (there are thousands) may have a different method for control. Leaf
diseases, in general, are controlled by the use of preventive foliar fungicides. The use
of resistant cultivars, and seed treatments (some rust fungi are seed-borne) for control
of certain rust diseases is beneficial. Most rust fungi are host specific. In the three
diseases described, for example, snapdragon rust only attacks certain genetic lines of
snapdragons. Hollyhock rust is similar. Limb rust of pine is also host specific. Within
each species of rust fungi there are pathogenic races that cause disease in only certain
cultivars within the host species.
These pathogenic races may vary from year to year and also be different in
other geographical areas. Snapdragons, for example, bred for resistance to rust in one
geographical area may be susceptible in other areas.
Cytospora Canker
Cytospora canker is commonly found on a number of fruit and shade
trees in Arizona. This disease occurs worldwide on fruit trees, hardwood, forest and shade
trees, shrubs and conifers. In Arizona the disease is most common on cottonwood (Populus
fremontii), lombardy poplar (P. nigra), pecan (Carya illinoinensis), and
willows (Salix spp.). A number of Prunus spp. including apricot (P.
armeniaca), peach (P. persica), plums (P. salicina), prunes (P.
domestica) and cherry (P. avium) are susceptible. Occasionally the disease has
been seen on apples (Malus communis). Over 40 pathogenic species of Cytospora
have been described worldwide. The ascomycetous sexual stage has been named Valsa
or Leucostoma. There is some confusion among authorities on the nomenclature of the
causal fungus. The asexual, pycnidial stage, Cytospora, is the most common stage
seen in Arizona.
Symptoms and Biology: Infections take place in above ground woody
tissue that has been damaged by frost, fire or sunburn, or through wounds caused by
pruning injuries and broken twigs and branches.
Typical symptoms of Cytospora canker are sunken lesions on infected
tissue. The cankers are perennial and continue to enlarge each year. The fungus slowly
invades and girdles limbs or trunks. The result is a dead limb above the infection site.
Black pycnidia of Cytospora can easily be seen emerging from infected bark with use
of a hand lens. The pycnidia are roundish and pinhead in size. They are scattered in the
cankered area. During wet weather, sticky masses of orange-yellow conidia are extended in
long tendrils. These conidia are wind disseminated to injured tissue where they germinate
and infect host tissue. Cytospora is active during spring and summer. The extruded
conidial stage is most commonly seen during our summer monsoon season. Inoculum can be
found on cankered trunks and branches throughout the year.
Control: Sun damage may be reduced by shading or application
of a white interior latex paint to woody tissue. Trees that are properly fertilized and
watered are not normally susceptible to infection. Avoid severe pruning. Disinfect pruning
cuts with a 1/10 dilution of a household bleach. Cut out infected
branches as they are a continual source of infection. Remove and burn where feasible.
There are no chemicals recommended for control. Removal of heavily infected cottonwoods
near commercial pecan groves has reduced disease incidence in some situations.
Sooty Canker
Sooty canker, also known as branch or limb wilt, is caused by the fungus, Hendersonula toruloidea. This wound pathogen invades only sunburned bark or areas that have been
mechanically injured, such as pruning wounds on smooth or thin barked deciduous trees. The
fungus does not infect uninjured host tissue. In Arizona, disease is found primarily at
low elevations where sunburn damage commonly occurs on trunks and branches of unprotected
or stressed shrubs and trees. The fungus has a wide host range and causes diseases in many
unrelated plants.
Symptoms: The most common symptom of sooty canker is the sooty,
black canker that develops beneath bark tissue. This black canker is due to the presence
of masses of black, fungal spores that appear under the bark and on the surface of the
canker. Symptoms on leaves of cankered branches appear during summer. Because the fungus
grows into and kills sapwood, the leaves on branches with cankers, wilt, turn brown and
die. Branches die back to the cankered area. Scattered branches are usually affected. Most
cankers develop on unshaded trunks or limbs that face toward the sun. Sunburned trunks and
limbs are highly susceptible to infection.
Figure 12. A close-up of a limb canker in cottonwood
caused by the sooty canker fungus, Hendersonula toruloidea. Note the
vlack masses of fungal spores that occur under the bark and on the surface of the canker.
Biology of the Pathogen: The fungus, under our conditions, produces
only conidia and thus has a very simple life cycle. The small conidia, produced in black,
powdery masses under bark, are easily wind disseminated. These spores, which arise from
segmented hyphae, are carried to damaged bark tissue where they germinate and initiate
infection. Most active fungal growth occurs during the summer. The mycelium grows into
living tissue. Infected sapwood becomes stained gray to black in color. Early research in
California simulated sunburn damage on bark of walnut trees with use of a
"blow-torch." This fact says something about our summer conditions.
Common susceptible hosts in Arizona include: almond (Prunus amygdalus),
apple (Malus sylbestris), apricot (Prunus armeniaca), ash (Fraxinus
spp.), carob (Ceratonia siliqua), Citrus spp. (oranges, grapefruit, lemon,
lime), cottonwood (Populus spp.), fig (Ficus cariaca), mimosa (Albizia
julibrissin), mulberry (Morus spp.), peaches (Prunus persica), pecan (Carya
pecan), plum (Prunus domestica), sycamore (Platanus occidentalis),
walnut (Juglans spp.), and wisteria (Wisteria spp.)
Control: Sooty canker can be controlled when infections are
confined to limbs and upper branches. Smaller infected branches should be removed when
symptoms appear. Since sunburned bark is the primary infection site, large limbs should be
pruned only when trees are dormant.
When removing infected limbs, cut back to at least 1 foot below the
canker. The cut area and pruning tools should be treated with a solution of 1 part
household bleach and 9 parts water. Pruning wounds should be painted with a copper
fungicide to prevent infection. Reapply the copper compound to the wound each spring to
insure adequate protection against infection. Control becomes increasingly difficult as
the disease progresses into the scaffold branches and is virtually impossible once the
main trunk is infected.
Tree vigor should be maintained through proper fertilization and deep
watering on a regular schedule. Severe pruning of larger branches and limbs of trees
susceptible to sooty canker should be avoided. Whitewash, applied to exposed lower trunk
areas, will reduce the possibilities of infection. This material reflects radiation and
reduces bark temperature.
Wood Rots and Decays
Wood-rotting basidiomycete fungi are occasional problems in desert plants
including cliff rose (Cowania spp.), canotia (Canotia holacantha), palo
verde (Cercidium spp.), creosote bush (Larrea tridentata) and junipers.
Also, some cacti such as saguaro and cholla which have a large amount of solid, woody
tissue may be invaded and decayed by wood-rotting Basidiomycetes as well as a number of
landscape trees, including mulberry (Morus spp.), cottonwood and poplars (Populus
spp.), and chinaberry (Melia spp.). Wood-rotting Basidiomycetes are common disease
fungi in a large number of conifers, deciduous trees, shrubs, and hardwoods throughout the
forest areas of Arizona. Large losses of timber on living and harvested trees are caused
annually by these fungi. Weakening of limbs and the subsequent hazard from breakage of
large limbs and trunks , especially on windy days or during wind storms, is a serious
consequence of these infections.
Figure 13. A shelf-shaped mushroom of a wood decay
fungs (Ganoderma sp). A cross-section of this area would reveal the dark, rotted heartwood where the fungus is active (photo courtsey of Tom Kruk).
Symptoms: In living trees and shrubs most rotting is confined to
the older, central wood of stems and branches. The conks of these fungi appear near the
infection site. These shelf-shaped fungal structures are most commonly seen along the main
trunk and on infected branches. Cross sections of infected wood reveals the dark,
discolored heartwood where the rot fungus is active. The fungus may be isolated from these
tissues.
The fungi enter through wounds. Shelf-shaped or bracket-like structures
usually occur after decay has been active for long periods of time. The disease progresses
slowly and generally is not noticed until the fungus has destroyed heartwood.
Control: Control consists of removal of infected tissue and
prevention of wounds to woody tissue. A specialist is necessary to identify these diseases
and to give advice on specific control measures. In any case, affected limbs should be
pruned away to avoid damage or injury if they fall.
Ganoderma Root Rot
Another basidiomycete, Ganoderma lucidum, causes root rot in native
and landscape trees such as olive, African sumac, mulberry, hackberry, and oak. Another
Ganoderma species causes root rot in mesquite and Acacia.
Biology and Symptoms: Spores of these Ganoderma species
infect the roots of susceptible hosts, and as the fungus grows in host tissue, it causes
root rot. Infected trees decline rapidly and die. The entire crown is affected as leaves
fail to emerge. These fungi seem to be limited to the roots and lower crown. They are soil
borne, and spread from plant to plant by root contact. The creamy colored mycelium of the
fungus can be seen if the bark is pulled away from the root. During summer rains, the
reproductive structures of these fungi may form on the base of the tree. These are white
to reddish-orange fleshy growths on the bark that become hardened when they mature.
Brownish-red spore deposits may be visible around the structures. These reproductive
spores are easily carried in the wind.
Control: There is no control for Ganoderma root rot
once the plant is infected. The best way to prevent infection is to prevent wounding.
Cutting roots during construction or landscape maintenance should be avoided wherever
possible.
Bacteria
The desert environment of Arizona is not conducive to bacterial plant
diseases. Foliar and above ground diseases that are of great importance in the wet, humid
areas of the world, are rare and insignificant. There are some exceptions, however. Two
gall diseases, crown gall and oleander gall are common. Also, fire blight, bacterial
necrosis of saguaro, and bacterial wetwood or slime flux are significant enough for
discussion in this publication.
Crown Gall
Crown gall is caused by the soil-borne bacterium, Agrobacterium
tumefaciens. This disease is one of the most widely studied of all plant diseases.
This gall inducing bacterium enters plants through wounds primarily on lower stems and
trunks and roots. Cellular growth is stimulated and galls of varying sizes and shapes are
produced in infected plant tissue. Agrobacterium tumefaciens has the largest host
range of any bacterial plant pathogen. More than 600 plant species in over 90 plant
families are susceptible.
Symptoms: Galls form on stems and roots, especially at the root
crown, the area where roots and stem come together. Galls enlarge with host plant growth.
Gall formation does not occur in dormant plants. The galls range in size from less than
1/2 inch in diameter to over 8 inches in susceptible, older plants. The size of the galls
is influenced by plant species and size and growth rate of the infected plant part. Galls
on woody plants are spongy and light colored when young but with age become rough, hard
and fissured. Old galls are often sloughed off.
Plants affected most commonly in Arizona include almonds, apples,
apricots, cherry, cottonwood, figs, grapes, nectarine, peaches, pears, pecans, plums,
privet, pyracantha, roses, and willows.
Biology: Strains of Agrobacterium can be isolated
from soil, galls, and root surfaces. Crown gall bacteria are disseminated in soil, water
and on or in plants. In grapes, for example, the bacterium becomes systemic in xylem
tissue. In Arizona the most serious cases of crown gall have occurred where early spring
growth is followed by freezing temperatures. This is more common at elevations above 3000
feet. This occurs because the bacterium is a wound pathogen. Agrobacterium tumefaciens
survives in the soil as a root colonizer of many plants without regard to their
susceptibility to infection and gall formation. When susceptible hosts are planted into
infested areas the bacteria infect through wounds and galls are produced. The bacteria are
returned to the soil when galls disintegrate, thus completing the simple life cycle.
Control: It is very difficult to purchase disease-free
plants because early galls are often too small to see or have not formed. Once the
bacterium is introduced into your landscaping area for all practical purposes it is there
indefinitely. The greatest economic losses caused by crown gall occur in nurseries that
produce stone and pome fruit trees such as almond, apple, cherry, peach, and plum because
regulations prohibit their shipment and sale. The damage caused by crown gall may have
been historically exaggerated. Recent studies in grapes, for example, suggest that only
very extensive galling causes reduced growth and yield loss. There is no practical method,
although several have been suggested, to eradicate the galls. If crown gall has been a
problem in the past, replant with resistant species such as palms, grasses or conifers. No
chemical methods are recommended. Avoid injuries during planting of bare rooted and other
ornamental plants.
A biological control may be available to prevent entry of
the pathogen. A strain of bacterium that attaches to a wound site but does not cause
disease can be used as a root dip or wound paint for preplant prevention. Development of
resistance of the pathogen to this strain has been a problem.
Oleander Gall
Oleander (Nerium oleander) is one of the most popular evergreen
shrubs in Arizona. The gall disease is widespread.
Symptoms and Biology: Oleander gall is caused by the bacterium Pseudomonas
syringae pv. savastanoi. Galls occur on twigs, branches, leaves, flowers, and
seedpods. Initially galls appear as small protuberances that subsequently develop into
wart-like growths with roughened, fissured surfaces. Galls vary in size but average about
1/2 to 1 inch in diameter. Large galls are usually made up of several small galls that
have grown together. Galls are the result of the growth and multiplication of the
bacterium. The bacteria enter and infect oleanders through leaf and blossom scars, wounds
produced by pruning, frost injury, and natural openings. Rain, sprinkler water, and
pruning tools can spread bacteria from diseased to healthy plants.
Control: When purchasing oleanders, examine them carefully
to be sure they are free of galls. The vast majority of nursery stock is free from disease
but prevention is always the most effective method of disease control. For diseased
oleanders, prune out infected plant parts and apply disinfection solution (a 10 percent
solution of household bleach) to each cut surface.
Always dip pruning tools in the disinfectant solution between cuts to reduce the
possibility of spreading the bacteria. Pruning operations should be conducted during the
dry seasons to avoid infection of wounds. Avoid sprinkler irrigation.
Severe infection of large shrubs is difficult to control by selective
pruning. If the entire shrub is cut down, the new succulent growth is extremely
susceptible to infection. In certain situations, removal of the diseased plant and
replanting may be the best method of control.
Fire Blight
Fire blight, caused by the bacterium Erwinia amylovora, is one of
the oldest known and most serious bacterial plant diseases. Interestingly all susceptible
plants are in the rose family (Rosaceae). The most important groups include apples
(Malus spp.), pear (Pyrus spp.), cotoneaster (Cotoneaster spp.),
hawthorn (Crataegus spp.), quince (Cydonia spp.), loquat (Eriobotrya
japonica), Prunus spp. (including apricot, cherry, plum, prunes), pyracantha (Pyracantha
spp.), mountain ash (Sorbus spp.), and spirea (Spiraea spp.). In Arizona
most reports of fire blight relate to apples, pears, and pyracantha. The causal bacterium,
E. amylovora, is thought to be indigenous to North America. The disease was first
reported on apples, pears, and quinces in the Hudson River Valley of New York in 1780. In
1878, an American plant pathologist, Thomas Burrill, working at the University of
Illinois, concluded that the disease was caused by a bacterium. The disease was not
described in Europe for another 100 years.
Figure 14. Symptoms of fire blight on loquat. Note the
infected brownish shoot tissue.
Symptoms: Extensive studies on symptom development, basic biology
and control have been made on apples and pears. Fewer studies have been made on other
hosts including ornamentals and native plants. In Arizona fire blight is a problem in
Cochise and Graham counties on pears and apples. The disease is occasionally a problem on
loquat, cotoneaster, and pyracantha in urban situations. Our desert environment does not
favor this disease in normal rainfall seasons.
Disease symptoms first appear when trees or shrubs are blooming. Blossom
blight, an early symptom, occurs because the bacteria first invade flowers. The pathogen
enters hosts through blossoms, wounds, and natural openings such as stomata, hydathodes,
lenticels and nectaries.
In Arizona, if high humidity, high rainfall and warm cloudy weather occurs
during the flowering cycle the pathogen can cause extensive damage. Infected blossoms of
pear, apple, loquat or pyracantha develop a water-soaked appearance, shrivel, and turn
brown to black. Affected flowers may or may not drop. Affected apple blossoms tend to
cling to the spurs. Young, rapidly growing apple shoots and twigs also are quite
susceptible. In the early stages of disease, reddish-brown streaking can be seen in the
tissues below visible infections. Affected apple shoots eventually turn light to dark
brown, while pear shoots turn dark brown to black. Affected leaves appear light to dark
brown.
Blackening of petioles and the central main vein of leaves is common.
Margins of advancing infections of pear fruitlets often have a dark green (water-soaked)
appearance, while those of apples are reddish.
The pathogen also can invade major branches and trunks of trees, causing
visible cankers. Cankers initially may have no obvious margins, or the margins may be
raised or appear as blisters. If a canker girdles a trunk, branch, or twig, the portion
above usually dies.
Under warm, humid conditions the bacteria can multiply so rapidly that
they will ooze from infected peduncles, shoots, leaves, fruits, and even cankers. Sticky
and light to amber colored, such ooze may appear as droplets, tendrils, or discolored
streaks. This is not a common phenomenon under Arizona conditions.
Disease Cycle: Erwinia amylovora survives
between season within and on host tissues. The bacteria do not survive in soil.
Temperatures between approximately 60° and 85°F,
humidity above 60 percent, and the frequent occurrence of free moisture (dews, rain, fog,
sprinkler irrigation) favor the multiplication and spread of the pathogen. Bacteria are
transferred from cankers to flower buds, new leaves, succulent shoots, and fruits by
splashing water and insects. The bacteria penetrate plant parts via natural openings,
including nectaries in flowers and stomata in new leaves and shoots. Penetration also can
occur through fresh injuries such as those caused by hail or chewing insects. Bacteria are
secondarily distributed from newly infected flowers and from ooze associated with
infections to non-infected flowers, and leaves by insects and splashing water. Pruning
tools also can be important in transferring bacteria from infected to non-infected
branches. Once infection of flowers or leaves occurs, bacteria can move into shoots, and
then upward and downward in the shoots without causing any external evidence of disease.
In fact, bacteria have been recovered approximately 35 inches beyond the margins of
visible lesions.
Control - Resistant varieties: Of the apple varieties
commonly grown in Arizona, Arkansas Black and Red Astrachan are currently considered
resistant. Golden Delicious, Red Delicious, Granny Smith, Jonagold, and Gravenstein are
moderately resistant to fire blight. Rome Beauty, Jonathon, and Lodi are susceptible to
the disease, as are most flowering crab apples. The predominant apple rootstocks used are
rated for fire blight susceptibility as follows: MM111, light; MM106, moderate; M9,
severe; and M26, very severe.
Currently, Bartley and Bosc pear varieties are considered highly
susceptible, Seckel moderately resistant, and Surcrop resistant to the disease.
Chemical control: Chemical control of fire blight is based
on protective sprays. Initiate spraying during the bud/bloom period when the average of
daily maximum and minimum temperatures reaches 60°F. Follow label
recommendations for mixing and applying sprays, including the timing for repeat sprays
(usually at five day intervals) during the blooming period, particularly when rain or hail
has occurred since the last application. Since any unprotected, open blossom is a
potential infection site, sprays should be continued throughout the bloom period when
environmental conditions exist which are favorable for blight.
Two groups of chemicals have traditionally been used for blight control.
These are:
- Copper compounds. Copper compounds can be effective in reducing fire
blight. They are easy to apply and relatively inexpensive. Although bordeaux (copper
sulphate plus lime) probably has been the most frequently used formulation, fixed coppers
such as kocide (copper hydroxide) and copper oxychloride sulphate (COCS) are much easier
to prepare and use as dormant and blossom sprays. However, copper can cause russet-ting of
fruit which can decrease their fresh market value.
- Antibiotics. Streptomycin has been the antibiotic of choice in
protective sprays. Although more expensive than copper compounds, streptomycin normally
does not cause russetting. However, resistant strains of E. amylovora have
developed in areas where streptomycin has been exclusively and repeatedly used.
Pruning/Sanitation: In the spring, remove infected shoots
and flowers. Prune them off at least 15 inches below any visible symptoms. Pruning
equipment should be decontaminated after every cut by treating the blades with 10 percent
household bleach. Because of their susceptibility to E. amylovora, promptly remove
unwanted suckers from trunks, roots, and scaffold branches. During the dormant period,
remove all remaining blighted plant parts. All infected prunings (summer and winter)
should be burned.
Bacterial Necrosis of Saguaro
This disease is caused by soft-rotting species of Erwinia. The
pathogen has also been recovered from a number of indigenous cacti including cholla,
prickly pear, barrel, and organ-pipe cacti.
Symptoms and Biology: Symptoms appear at one or more positions on
the trunk or branches of saguaros. The first indicator of bacterial necrosis is a circular
darkening and softening of the plant tissues. The infected area enlarges, becoming
purplish-black, and splits open. A dark odorous material will frequently "leak"
from the plant. At other times, the soft areas dry and crack, revealing the dark, dry
remains of diseased tissues. If conditions are favorable, the plant can confine the
disease to a "pocket" by forming a barrier of protective tissue (callus) around
the infected area. If this tissue does not rapidly form, the bacteria will spread.
Figure 15. Symptoms of bacterial necrosis of saguaro. Note the black ooze coming from the infection site above the two arms.
Bacteria occur in the diseased plant tissue of living cacti, and in the
exudate associated with the infected areas. Infection begins when the pathogen is
introduced into the cactus through wounds or natural openings. Presumably, insects and
small animals which are associated with diseased or decaying saguaros serve as the
principle carriers.
Control: Removal of diseased tissue is "the
most practical way to control the spread of the infection within the
plant. If the affected area is small, remove all the rotting material
and about one-half inch of the surrounding healthy tissues. Slope the
bottom of the excavation so that water will drain out. The wall of the
cleaned rot pocket should be smooth. In removing the rotting material,
be careful not to make "puncture-type" cuts into the remaining
healthy tissue since the pathogen can survive in such sites. Thoroughly
wash the cleansed pocket with a 10 percent household bleach solution
(1 part household bleach and 9 parts of water; also include about one
teaspoon of liquid detergent per gallon of solution) and then allow
the pocket to stand open to hasten healing.
If the lesion is so large as to nearly girdle or weaken an arm or plant,
serious consideration should be given to removal of the affected structure. Otherwise,
damage might result should the branch or plant unexpectedly fall.
Wetwood or Slime Flux
These two diseases are described under bacterial diseases because a number
of diverse bacteria, including Erwinia spp., are thought to be the cause of these
poorly understood diseases. In the literature the disease is referred to as slime flux
and/or wetwood.
Symptoms: Symptoms of Slime Flux consist of water-soaked,
discolored areas at or below branch crotches and trunk wounds with chronic bleeding of
sap. Wilting and die back of branches may occur. Water-soaked wood with large numbers of
bacteria is discolored and dead. Liquid may seep out of cracks and wounds and run down the
bark. The liquid, because of contamination with microorganisms, becomes dark in color,
sticky and odiferous. Fermentation of tree tissues may cause increases of pressure and
toxin production within the infected tree. Normally, the disease is not found in young
trees. This is probably due to the fact that in the sapwood and heartwood of normal, young
actively growing trees, bacteria and fungi are rare. Susceptible hosts in Arizona include:
ash (Fraxinus spp.), elms (Ulmus spp.), cottonwood (Populus fremontii),
mulberry (Morus spp.) and mesquite, common and chilean (Prosopis juliflora
and P. chilensis).
Control: There are no preventative methods for Slime
Flux except good tree health care practices, proper watering, feeding and pruning. There
are no controls for the disease. The practice of installing tubes to drain liquid is no
longer recommended since it does not alleviate the problem and the holes are a good
infection site for many pathogenic organisms. Trees with Slime Flux will usually live for
many years, but any weakened limb should be removed if it is a safety risk. If the
dripping causes stains on patios or walkways, a hard water spray, applied routinely, may
prevent staining.
Nematodes
It is beyond the scope of this publication to go into detail concerning
diseases caused by nematodes. There are hundreds of species that parasitize plants. Most
plant-parasitic nematodes attack only roots. Symptoms on roots cover the range from
swelling, lesions, galls, and suppression of root elongation. Above ground symptoms are
non-specific and consist of slow growth, chlorosis and all symptoms associated with root
damage and nutrient deficiencies. Specific diagnosis requires the analysis of root and
soil tissue to determine qualitative and quantitative populations of nematodes.
Root-Knot Nematodes (Meloidogyne spp.)
These nematodes are the most common and important group.
They cause disease in hundreds of plants including ornamentals, vegetables
and the following commonly planted landscape trees: Acacia (Acacia
spp.), almonds (Prunus spp.), apricots (Prunus spp.),
Arizona ash (Fraxinus velutina), Chinese elm (Ulmus parvifolia),
fig (Ficus carica), peach (Pyrus spp.), pear (Pyrus
communis), plum (Prunus spp.), pomegranate (Punica granatum),
weeping willow (Salix babylonica), golden willow (Salix alba
`Tristis'), bottle tree (Brachychiton populneus), olive (Oleaeuropaea),
palm (Phoenix canariensis and P. dactylifera, Washingtonia
filifera and W. robusta, Chamaerops humilis, Arecastrum romanzoffianum,
Trachycarpus fortunei), and Japanese privet (Ligustrum japonicum).
Figure 16. Root-knot nematode on roots of an annual,
herbaceous plant. Note the galls of varying sizes in the roots.
Symptoms: Plants are stunted with small, pale green or
chlorotic foliage that tends to wilt and partially defoliate in warm weather. Flower
production and fruit set are reduced. Irregular, spherical swellings (knots) are found on
the roots at the point of nematode feeding.
Control: Control is based on first identifying the nematode.
Then it is possible to implement certain cultural techniques such as the use of resistant
plants and varieties, rotation, and fallow. Purchase plants from reputable nurseries and
producers.
Parasitic Higher Plants
Over 2500 species of higher plants live parasitically only on other
living plants. The most important genera worldwide include dwarf mistletoes of conifers (Arceuthobium
spp.), mistletoes on broadleafed trees (Phoradendron spp.), the European true
mistletoes (Viscum spp.), broomrape (Orobanche spp.), witchweed (Striga
spp.), and dodder (Cuscuta spp.). All of these parasitic plants produce leaves,
stems, flowers, and seeds but no true roots. The mistletoes have chlorophyll but depend on
their hosts for water and nutrients. Others, like dodder, have little or no chlorophyll
and no true roots. The dodders are totally dependant on their hosts for completion of
their life cycles.
Mistletoes
In Arizona, although dodder and broomrape are occasionally seen on a
number of broad-leafed plants, the most important and common parasitic plants in this
group are the mistletoes. Two groups are important, the dwarf mistletoes of conifers and
the true mistletoes of broadleafed trees and shrubs. Dwarf mistletoes are common problems
in our conifer forests.
In urban plantings in the desert areas of Arizona the only significant
mistletoes are the true mistletoes caused by Phoradendron spp. Several Phoradendron
spp. have been described in Arizona including P. californicum (parasitic on
leguminous shrubs and trees such as acacia (Acacia spp.), blue palo verde (Cercidium
floridum), foothill palo verde (Cercidium microphyllum), Mexican palo verde
(Parkinsonia aculeata), ironwood (Olneya tesota) and mesquite (Prosopis
spp.). Other Phoradendron spp. occur on juniper (Juniperus spp.) and oak (Quercus
spp.).
The most conspicuous and largest leaved mistletoe in Arizona is P.
tomentosum which parasitizes cottonwood (Populus fremontii), sycamore (Platanus
wrightii), ash (Fraxinus spp.), hackberry (Celtis spp.), walnut (Juglans
spp.), and willow (Salix spp.). It is often a serious problem along rivers and
streams, washes and in parks and golf courses with large cottonwood trees.
Symptoms and Biology: One of the most common sights in areas
where these hosts grow is the green, straggly, bushy growth of mistletoe hanging from
infected branches. The true mistletoes are spread by birds that eat the berries and
deposit the viable seeds with their feces. Seeds germinate on the host plant and cause new
infections.
Often the mistletoe completely takes over the plant and death occurs.
This is a long process. Heavily infected desert shrubs and trees have been infected for
many years. Mistletoe plants are often so numerous as to obscure the host plant. Phoradendron
spp. are perennial evergreens. Thus, deciduous trees, like cottonwoods, appear almost as
evergreens during the winter if infection is heavy.
Infected branches and stems become swollen. The mistletoe itself grows
into the plant well beyond the visible its growth on the plant. Slow, progressive decline
and death of branches may occur. Infections on lower trunks, which are fairly common in
desert legumes, are more damaging to the tree than infections that occur on upper
branches. Dead or dying branches are weakened and become susceptible to breakage.
Control: Mistletoes are very difficult to control. The only
practical method of control is to prune out infected branches approximately one foot below
the infected sites. Also, the mistletoe plant should be physically removed at least
annually in situations where pruning (lower trunk infection) may not be feasible.
Nonparasitic Diseases
Many plant diseases that occur in our diverse climate zones are not
caused by parasitic micro-organisms but are nonparasitic in nature. Soil conditions where
these nonparasitic diseases are common include high pH (alkaline soils), saline soils
(salt contents with electrical conductivity measurements of 4 mmhos per cm or greater from
a saturated soil extract, ECe), sodic soils (exchangeable sodium percentages, ESP of 15 or
greater), low soil organic matter content, poor soil moisture holding characteristics,
shallow soils, poor soil structure, and caliche deposits. Weather conditions contributing
to these diseases include intense summer heat, low humidities, drying winds, low rainfall
and unpredictable freezes. Other factors include inappropriate plant selection, poor
pre-plant soil preparation, inadequate or excessive fertilization, improper watering
techniques (particularly in drip irrigation systems) and damage caused by herbicides.
Some of our most common nonparasitic diseases are:
Alepppo Pine Blight
Symptoms:This physiological disease only
occurs on the commonly planted Aleppo pine (Pinus halepenis).
Death of needles, twigs, and branches occurs usually in trees over 5
years of age. Most symptoms develop in the upper part of the tree. Blighted
needles take on a gray-green color that later turns reddish-brown. These
needles may persist on the tree until their seasonal summer defoliation.
Some twig and small-branch death may follow needle blight.
Conditions Favoring Disease: Aleppo pines planted in shallow or
poorly draining soils are particularly subject to drought stress. Poor soil drainage
caused by underlying hard-pan contributes to blight by hampering root development. Drying
winds and low relative humidity during fall and spring are commonly associated with the
onset of symptoms.
Control: Maintain a uniform deep water supply to the tree
throughout the year. Irrigate so that water is available to a depth of at least 5 feet for
mature trees. Apply about 1/4 cup of nitrogen fertilizer per inch of the trunk in
diameter. Apply one-half the fertilizer in March and the other half in July.
Scorch
Symptoms: This disease occurs during hot, dry, windy
weather. It is characterized by marginal browning and drying of foliage. The roots of the
plant fail to take up sufficient moisture to make up for the heavy amounts of moisture
lost during the dry, hot period. Deep, thorough irrigations at 2 to 3 week intervals may
help prevent this problem. This symptom, caused by water stress during our hot, dry summer
period is common to many trees, shrubs and ornamentals.
Arizona Ash Decline (Fraxinus velutina)
Symptoms: Branches may exhibit symptoms of leaf tip and edge
burning. These symptoms are similar to those associated with drought or excess salts.
Leaves are usually small, and elongation of internodes is reduced, causing a tufted or
rosette appearance. Dead leaves often remain attached to the affected limbs. Dieback
becomes noticeable beginning about July. As the decline progresses entire branches and
limbs die until the whole tree is killed.
Conditions Favoring the Disease: The disease has been observed
throughout the state wherever ash trees are cultivated (at elevations ranging from 1,300
to 5,700 feet). As a native of southwestern Arizona, Arizona ash is relatively tolerant of
drought and alkali. Well fertilized, deeply irrigated ash trees appear more able to
compensate for the dieback and remain lush in spite of being affected.
Control: Maintaining trees in a vigorous
condition will help reduce the rate and severity of decline. Frequent (weekly to biweekly
for mature trees), deep irrigations during the hot summer months and at least monthly
irrigations in the cool seasons are recommended.
Mulberry Tree Decline (Morus
spp.)
Symptoms: Symptoms consist primarily of small leaves and dieback
of the upper twigs and branches. The disease usually appears in trees that are 15 to 20
years of age. The overall tree decline is slow, but death may occur 2 or 3 years after the
initial symptoms.
Causes of Disease: There are many possible factors involved in
any tree decline disease. One of the major factors, however, is insufficient watering,
particularly in trees that are in lawn situations.
Control: The mulberry tree needs infrequent (once a
month) deep (4 feet) irrigations particularly during the summer months. Tree wells should
extend to the drip line of the tree.
Oleander Decline (Nerium spp.)
Symptoms: Because of the stiff, leathery leaves and woody stems
and branches, wilting is not commonly observed in oleander. Instead, in response to water
stress, foliage turns dull gray-green in appearance and becomes brittle. Lower leaves turn
yellow, dry, and drop off.
Control: If a watering basin is used, it should be
three to four inches deep and extend out from the base of the plant to the drip line.
Basins should be filled at each irrigation in order to saturate soil to a depth of at
least three feet.
These 5 diseases are examples of water stress conditions. The symptoms are
similar in most of the non native, introduced shrubs and trees. This disease is often
noticed in poorly designed drip irrigation systems. Superficial sprinkling, instead of
deep basin waterings, are a major factor in these problems.
Salt Problems
The most common nonparasitic disease in our desert areas is damage
caused by excessive soil salts. Factors that increase salt accumulation in our desert
soils include: poor drainage caused by dense subsoil layers and caliche deposits,
inadequate leaching which allows salt accumulation in the rooting area, excessive salt in
irrigation water, low rainfall and high temperatures, and excessive applications of
fertilizer and manure. Symptoms of salt damage include plant stunting, marginal burning of
leaves, and premature defoliation. Salt tolerance of urban plants to salt damage varies
greatly. Symptoms of salt damage are nonspecific.
There are many causes of the above
described symptoms including root diseases, insects, mineral deficiencies and excesses,
and herbicide damage. Diagnosis of salt problems requires the taking of a soil sample in
the root area of the affected plant. Tissue analysis may also be necessary.
Miscellaneous Problems
Other nonparasitic problems include a wide range of conditions
from lightening damage, frost and freezing damage, sun scald, nutrient deficiencies,
pesticide damage and air pollution. Professional help is necessary to determine the cause
of many of these problems. Some, however, like lightening damage, frost damage, extreme
water stress, and pesticide damage, can be diagnosed with only the use of a little common
sense. Nutrient problems, salt damage, and air pollution require professional help for
diagnosis and control.
|