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Plant Pathology


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  MG Manual Reference
Ch. 4, pp. v - x
[ Introduction: history | diseases defined| conditions | symptoms | development | control | summary ]


A symptom is the physical expression of a change in the appearance and function of the plant. Examples of symptoms are:
sudden death of twigs, foliage, flowers.
dead places on bark and cortex of stems; often discolored and raised or sunken.
abnormal, localized swellings on leaf, stem, or root tissue.
general decomposition and destruction of tissue.
death of tissue.
circular or irregular lesions on above-ground tissue.
A sign is the visible presence of the pathogen, such as a fruiting body or discharge associated with the disease:
fungal fruiting structures formed on rotting woody plants (shelf or bracket fungi).
masses of fungal threads (hyphae) which compose the vegetative body of the fungus.
Ooze (flux)
viscid mass of juices composed of host and parasite substances found exuding from some diseased plants.
minute, fungal, asexual fruiting structures, usually globose and black, formed on plant surfaces.
string-like strands of fungal mycelia sometimes found under bark of trees.
A disease syndrome is the group of signs and symptoms which collectively characterize a disease. Familiarity with a disease's signs or symptoms is not enough to diagnose a disease; it is necessary to know the syndrome and case history. Seeing a spot on a leaf doesn't tell you much, but finding pycnidia in that spot and knowing the plant species and recent weather conditions might be sufficient information to diagnose the disease. Other times, laboratory work is necessary for diagnosis.


It is important to understand how plant diseases develop in order to control them. By the time it becomes obvious that a plant has a disease, it is generally too late to do anything about it in that growing season. Plants cannot be cured in the way people expect their own ills to be cured. The process by which diseases develop can be broken into five distinct phases:
This is the introduction of the pathogen to the host plant tissue. Wind, or rain, or running water can move pathogens and introduce them to a host plant, as can birds, insects, people, or equipment. Some pathogens move themselves short distances, but most rely on other means. Sources of inoculum include plant debris, seed, perennial plants, and soil.
This is a period of development during which the pathogen undergoes changes to develop a form which can penetrate or infect the new host plant. Some fungi, for instance, grow a structure called a penetration peg that can grow through the cell walls of the plant.
This is the process of getting inside the plant. It may be an active or passive process. Some pathogens produce enzymes to dissolve the cutin and cellulose layers of plant material between them and the cell contents. Some pathogens can swim through water on a plant's surface and into the plant through natural openings (such as stomata, lenticels, or hydathodes) or through wounds. Some pathogens are put inside the plant by insects, pruning tools, or driving rain.
When the pathogen invades the plant tissue and establishes a parasitic relationship between itself and the host, infection has occurred.
When the host plant responds to the presence of the pathogen, a disease exists. The host's response results in symptoms of the disease, such as blight or necrosis. As the pathogen matures, it produces inoculum in the form of spores, virus particles, and bacterial cells that can be spread or disseminated to other adjacent, healthy plants.


The importance of understanding the disease development process becomes obvious when considering control options. By the time symptoms are expressed, the pathogen (with few exceptions) is already inside the host plant and is relatively safe. Therefore, control efforts, in most cases, must occur before penetration has taken place. The overall principle in effective disease control is to keep the inoculum density of the pathogen at very low levels.
Success in controlling plant disease will occur when a combination of the following methods of control are used:
A grower can avoid certain diseases by choice of geographic area or choice of planting site. Disease can be avoided by planting at a time that does not favor disease. Using disease-free planting stock or modifying cultural practices also helps to avoid disease.
A grower can inspect stock for signs of disease and reject or treat any which is suspect. Plant quarantines are designed to exclude certain pests from areas that are free of that pest. Elimination of carrier insects can exclude a disease.
Once a disease is established in an area, eradication is unlikely. However, significant reduction in disease inoculum can be attained by destroying diseased plants or alternate hosts, by rotating crops, or by soil solarization.
Spraying or dusting plants with fungicides or bactericides can protect them from disease. Sometimes modifying cultural practices or the environment may protect the crop. Control of carrier insects will also protect plants.
Breeding and selection are used to develop resistant crops. Resistance can be enhanced through proper culture of a crop. Tolerance is another form of resistance in which the plant becomes infected but goes on to mature and yield normally.
Several disinfectants can be used to surface sterilize growing surfaces. Bleach, in a 1:1 solution, is effective against viruses, bacteria, and fungal spores. It is not effective against sclerotia or other resting fungal structures. Alcohol may be used as a disinfectant of tools, but is not effective against fungal sclerotia, viruses, and some bacteria.
Removal of diseased parts of a plant will sometimes control the disease. Heat can be used to treat contaminated seeds and to eliminate viruses from certain types of fruit tree bud wood.
Familiarity with crops and the diseases and insects that affect them is useful in planning control programs. Some diseases occur every season; others occur sporadically. Some can be controlled easily by using proper methods; others must be tolerated. Knowing which problem falls into which category comes with experience. Knowing the proper method to use at the proper time is a part of integrated pest management.


Plant diseases are to be expected. Fortunately, there are few truly devastating diseases in most years.
For disease to occur, there must be a susceptible host, a suitable environment, and a living pathogen. When all three conditions are met, disease occurs. Severity of the disease depends on the degree to which the conditions are met.
Disease development follows a precise course of events. Inoculation occurs first, usually followed by incubation. Penetration of the host is next. Infection occurs when the pathogen invades the host tissue. Only when the host responds has disease occurred. By this time, it is usually too late to control the disease.
Control involves more than the use of chemicals for protection. Avoidance, eradication, exclusion, resistance, and therapy all have a role in disease control. A combination of these will give best results.

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