Apple Scab Venturia inaequalis
Symptoms
Yellow or chlorotic spots on leaves; dark olive green spots on leaves and fruits; there may be a velvety growth on the undersides of the leaves; twisting of leaves; distorted leaves; severely infected leaves turn yellow and fall from the tree.
Agent
Fungus
Disease Development Conditions
The fungus overwinters on dead leaves on the ground; spores dispersed by wind; high humidity encourages fungal growth.
Control Methods
Remove all fallen leaves from the tree in autumn and compost them to prevent any disease from living in the debris; application of zinc and fertilizer-grade urea in autumn may be necessary to accelerate leaf fall, then lime should be added to the fallen leaves; fungicide application may be necessary in areas where leaves remain wet for periods exceeding 9 hours; fungicides such as copper soaps and Bordeaux mixture should be applied if there is a possibility of wetting as soon as leaf tips emerge.
Black Rot Botryosphaeria obtusa
Symptoms
Purple spots or circular lesions that are brown in the center and purple at the edge; red spots, purple lesions and/or brown-black rings on fruits.
Agent
Fungus
Disease Development Conditions
Spores can overwinter in branches or fruits remaining on the tree and spread during rainfall.
Control Methods
Remove dead wood, mummified fruit, and cankers from trees to reduce the spread of diseases; burn prunings made from the tree; the disease can be controlled by applying fungicide from silver tip to harvest.
Apple Powdery Mildew Podosphaera leucotricha
Symptoms
White velvety patches on the underside of leaves; chlorotic spots on the upper side of leaves
Agent
Fungus
Disease Development Conditions
Fungal spores overwinter in buds; spread by wind
Control Methods
Pruning infected shoots while dormant in early spring; apply sprays at the pink bud stage to reduce buildup; organic treatments include the application of lime and sulfur
Apple Flyspeck Zygophiala jamaicensis
Symptoms
Shiny black fungal fruiting bodies appear as dots arranged in irregular to circular patterns on the fruit surface.
Agent
Fungus
Disease Development Conditions
Outbreaks usually follow above-average summer temperatures and frequent rainfall.
Control Methods
Pruning trees to open the canopy and encourage drying of the fruit surface; fungicides can be applied as a preventive measure.
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