Alternaria mali Roberts
The disease primarily affects the foliage, causing circular, necrotic lesions with a light brown interior (A) that later become surrounded by a darker purplish halo
(B). Defoliation can occur by late summer on susceptible varieties that are heavily infected (C). The pathogen can also attack green, woody tissue, but it rarely attacks the fruit.
An increasing problem in the eastern North America, but has yet to be found north of Virginia.
This disease can be confused with frog-eye leaf spot and with leaf spotting caused by numerous other pathogens, including cedar apple rust on rust-resistant varieties. Injury from azoxystrobin fungicide (Abound) may also appear similar, but injury from azoxystrobin is most severe on McIntosh, Cortland, Empire, and Gala, whereas Delicious leaves are not affected by azoxystrobin. Similarly, injury from the fungicide captan may produce similar symptoms (D). Alternaria blotch may sometimes be confused with magnesium deficiency. Isolation of the pathogen is usually needed to confirm diagnosis.
The disease occurs primarily on the variety Delicious. Management requires an integrated approach. Shredding leaf litter in the fall helps to reduce disease pressure the following season (this will also help at reducing apple scab). Controlling red mite populations is essential to achieving control of Alternaria blotch. The strobilurin fungicides are the most effective at controlling Alternaria blotch.
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