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Alternaria Leaf Blotch

Alternaria leaf blotch is caused by the fungus Alternaria mali.  It affects apples, and it is mainly encountered in southern states.


Purple-brown leaf spots, defoliation.

Organic Treatments

Alternaria infections are associated with mites, and these must be controlled with dormant spray oil 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.

Conventional Treatments

Controlling mites from petal fall to mid June and keeping the ground tightly mowed will help keep this disease from doing economic damage. Dormant oil at a 2% rate from ½ inch green to tight cluster will be very effective at controlling mites.

Disease Cycle

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.

For more information, see the North Carolina Extension Factsheet on Leaf Spots of Bother: The Troublesome Three.

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