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Plant Pathology


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  MG Manual Reference
Ch. 4, pp. 18 - 20
[ 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 ]


Premature fruit drop • Natural thinning • Trees may produce more fruit than they need and thin themselves naturally
• Spring frost • Frost often kills developing fruits or buds
• Poor pollination • Tree may require other trees nearby to pollinate it; be careful not to kill bees with insecticides
• Environmental stress • Drought, cold, or heat can cause fruit drop
Poor fruit development (small number of fruit on tree) • Poor pollination • Tree may require other trees nearby to pollinate it; be careful not to kill bees with insecticides
• Biennial bearing Apples and pears naturally bear a heavy crop one year and few fruits the following year
• Improper pruning • Do not prune off fruit bearing wood during the dormant season; consult pruning manual for proper instructions on pruning
• Frost injury  
Fruits too small • Failure to prune • Peaches, nectarines, plums, and apples tend to produce many small fruits if not pruned; consult pruning manual for proper pruning
• Failure to thin fruit • Thinning fruit is necessary on some fruit trees
• Poor soil fertility • Amend soil as needed
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
Oozing sap on branches or trunk • Natural gummosis • Cherries, plums, apricots, and peaches naturally ooze sap
• Environmental stress • Drought or waterlogging can use fruit trees to ooze excessively
• Disease or insect damage • See section on specific problems
• Mechanical injury  
Large areas 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; follow proper pruning techniques
• Fungal or bacterial canker (any of several) • Same as for secondary decay
Brown dead areas on leaf margins • Leaf scorch • Water tree deeply during dry periods; scorch is usually caused by hot, dry weather, but root rots or other root damage can also be involved
Interveinal yellowing of leaves; no wilting • Iron deficiency • Apply chelated iron solution to foliage or soil, depending on formulation and tree species
• Waterlogged soil • Improve drainage
Young leaves curled and distorted; clusters of insects on undersides of leaves • Aphids • Use registered insecticide or hard stream of soapy water; thorough coverage of underside of leaves is necessary
Silk tents in branch crotches in spring • Tent caterpillar • Physically remove tents or use registered insecticide when caterpillars are small
Leaves with tiny white spots, often dirty with webbing • Spider mites • Use registered miticide or spray foliage regularly with water
Bark encrusted with tiny, slightly raised bumps; apples may have red spots with white centers • San Jose scale • Use a dormant oil spray
Internodes shortened, giving foliage a bunched appearance; yellowish mottling of leaves; shoot dieback • Zinc deficiency • Apply foliar application of zinc in fall before leaf drop; summer zinc applications on fruit trees may cause defoliation

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