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Alternaria Leaf Blotch
Also SeeFruit Tree Diseases and Pests: An Introduction
List of Common Fruit Tree Diseases and Pests
Alternaria leaf blotch is caused by the fungus Alternaria mali. It affects apples, and it is mainly encountered in southern states. The fungus overwinters in dead leaves, infected twigs, and dormant buds. It becomes active about a month after petal fall, and it is most prolific in wet weather when temperatures range between 75°F and 85°F. Infection will occur if surfaces are wet for 5.5 hours or longer.
Infected leaves will develop "frogeye" spots, starting as small purplish round spots and developing into larger, irregular brown spots with dark borders. When infection occurs on petioles (leaf stems), the leaf will yellow and drop. This disease can be confused with black rot, but unlike black rot it will not affect fruit and it will not cause cankers. The primary concern with alternaria is severe defoliation and consequent weakening of the tree if infection levels are high.
Alternaria infections are associated with mites, and these must be controlled if alternaria is present. Removing debris and mowing or burning fallen leaves is always good practice when it comes to fungal diseases. If you live in an area where alternaria is a problem, consider planting resistant varieties such as Granny Smith, Jonathan, Jonagold, Mutsu (Crispin), McIntosh, and Gala. In some cases, an application of fungicide may be necessary.
For more information, see the North Carolina Extension Factsheet on Leaf Spots of Bother: The Troublesome Three.
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