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Plant Pathology


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  MG Manual Reference
Ch. 4, pp. 25 - 28
[ Diagnostic Key: vegetables | specific vegetables; asparagus, bean, beet, carrot, cole crops, corn, cucurbits, eggplant, lettuce, onion, pea, pepper, potato, tomato | tree fruits| specific fruits; apple, stone, citrus | ornamentals | specific ornamentals; rose family, rose, palm, pine ]


Rapid wilt and death of plant with dead or dying foliage remaining attached; roots of larger trees are decayed and brown • Texas root rot (fungal disease) • No chemical control is available; see Extension bulletin for care of affected plants
Many small twigs broken off • Twig pruner, twig girdler (insects)
• Wind breakage
• Rake up and destroy broken and fallen twigs
Large area of split bark; no decay evident • Frost cracks • Frost can split tree trunks if sap in trunk expands; use tree wrap or tree paint to protect bark from sun and extremes in temperature
• Sunscald • Thin-barked trees, e.g. young ones, split when exposed to intense sunlight; use tree wrap or tree paint, especially during winter months
• Mechanical injury, e.g. lawn mower • Dig up grass around trunk and replace with mulch to avoid mowing too closely to base of tree
• Lightning injury  
Large areas of split bark; decay evident in wood • Secondary decay of wounds • No adequate controls; remove loose bark; practice proper pruning techniques
• Fungal canker (any of several) • Same as for secondary decay
General browning of conifer needles • Drought • Water deeply
• Salt injury • Irrigate to a depth of at least 3 feet to flush salts from root zone
• Gas leak • Check soil around roots for gray, crumbly appearance and foul smell indicative of gas leak
• Waterlogged soil • Improve drainage
• Transplant shock • Water regularly after transplanting
• Girdling roots • Be sure main roots are not wrapped when transplanting; may be necessary to cut rootball in several places before transplanting
Sour-smelling sap oozes from cracks in bark and/or from old pruning wounds • Slime flux, caused by microorganisms growing in sap • Provide proper watering and fertilization; no chemical or mechanical control
Shelf-shaped fungal structures produced along main trunk and branches; cross-sections reveal dark, discolored heart wood • Heart rot and wood rot (fungal diseases) • Destroy brackets to prevent spore release; remove badly infected trees; prevent infection by proper pruning technique
Branch swellings, witches' brooms, progressive decline and death of affected branches • Mistletoe • Prune out infected branches 1 foot or more from infected sites in the direction of the trunk
Yellow and green mottle or mosaic pattern on leaves; leaves may be distorted • Virus disease • Removal of plant may be necessary if virus is easily spread
Foliage becomes sparse; limbs produce smaller than normal leaves; dry, paper-thin bark cracks and splits to reveal a black, dusty mass of fungal spores • Sooty canker (fungal disease) • Prune out affected branches at least 4 inches below canker; disinfect pruning cuts with dilute bleach solution; destroy pruned wood to prevent further infection
Twigs and branches die back; small black fungal structures embedded in the dead outer bark Cytospora canker (fungal disease) • Prune out infected branches; disinfect pruning cuts with dilute bleach solution; destroy pruned wood to prevent further infection
Oozing sap on trunk • Natural gummosis • Some trees naturally ooze sap
• Environmental stress • Drought or waterlogging can cause trees to ooze excessively
• Mechanical injury • Prevent lawn mower injury
Leaves chewed or completely eaten • Various caterpillars, sawflies, leaf beetles, grasshoppers, etc. • Use registered insecticide while insects are small and before damage is extensive
Young leaves puckered, curled and distorted; clusters of small insects on undersides of leaves; clear sticky substance on leaves • Aphids • Use registered insecticide or hard stream of soapy water; thorough coverage of underside of leaves is necessary
Galls (abnormal growths) on leaves, stems, or branches; common on oaks • Various insects or mites • There are no chemical controls for gall insects, but the plants will not be seriously harmed
Proliferation of branches at specific points on the plant, forming a witches' broom effect • Insect injury
• Fungal, viral, or mycoplasma disease
• Mistletoe
• For all of these, only control is to prune out affected areas
Gray-white powdery growth on leaves; leaves and fruit may be distorted • Powdery mildew (fungal disease) • Use registered fungicide; prune out distorted, mildewed twigs; wettable sulfur is an effective preventative; do not apply sulfur when temperatures exceed 90 degrees F
Brown, gray, green, or yellow crusty, leaflike growths on trunk and branches • Lichens • Lichens are a combination of algae and fungi; they grow in moist, shady areas and do not harm the plant
Early leaf drop • Environmental stress, such as drought, compacted soil, or transplant shock • Provide proper culture
• Various insects or disease  
Brown dry areas on margins of leaves; some species may have irregular brown spots on leaves • Scorch, caused by hot dry weather • Water tree deeply
• Salt burn • Leach soil by irrigating soil to at least 3 feet

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