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Alternaria Blotch

Symptoms

Leaf spots appear on the leaves in late spring and early summer. Initially, they are 1/8 to 1/4 inch in diameter, round, brown, and occasionally have a purple border (fig. 1). As spots age, they often turn tan to ash gray (fig. 2). Some spots undergo secondary enlargement, becoming irregularly shaped (fig 3). Heavily infected leaves often abscise, resulting in defoliation. (Defoliation is greater when mites are present.) Fruit infections result in small, dark, raised pimple-like lesions associated with the lenticels. Twig lesions, which are somewhat sunken, round, blackish spots bordered by cracks, occur on susceptible cultivars such as Indo but have not been observed on Delicious.(Alternaria blotch looks very similar to frogeye leafspot and captan injury [fig. 4 and 5]. Often Alternaria blotch can be distinguished by its distribution pattern in the tree. Frogeye leafspot is usually clumped and associated with dead wood or mummified apples. Captan injury is usually associated with a set of 2 to 4 leaves on terminals, representing a specific spray event. Alternaria blotch, however, tends to be uniformly distributed on foliage throughout the tree, though it may be more severe in the lower third where the foliage remains wetter.)

Fig 1 - early
Fig 2 - later
Fig 3 - Irregular enlargement
Fig. 1. Early
Fig. 2. Later
Fig. 3. Irregular enlargement
Similar Lesions:
Fig 4 - Frogeye leafspot
Fig 5 - Captan injury
Fig. 4. Frogeye Leafspot
Fig. 5. Captan Injury

Disease Cycle

Alternaria mali primarily affects leaves of Delicious and cultivars with Delicious as a parent (e.g. Empire). Golden Delicious is moderately resistant but becomes infected when planted as a pollenizer in orchards of Delicious. Redgold, Fuji, Mutsu, Jonagold, and Jonathan are also susceptible, but to a lesser degree than Delicious. The fungus primarily overwinters on fallen leaves on the ground, but it can also overwinter in dormant buds. Primary infection occurs in late spring, and numerous secondary infections occur throughout the summer in hot, rainy weather. Infection can occur in 5 to 6 hours at favorable temperatures (70 to 75 degrees F). Severe infections result in up to 50% defoliation of trees by midsummer and a reduction in fruit yield and quality.

Defoliated trees (on left)

Fig. 6. Defoliated trees (left)

Control

It is very important to maintain mites below 10 per leaf in orchards with a history of the disease in order to minimize defoliation. Shredding leaves or applications of urea to leaves just before or after leaf fall may reduce the inoculum. The strobilurin fungicides Flint and Sovran have shown good activity on the disease when applied in a sequence of three sprays. A threshold for beginning the applications has not been established, but the disease incidence should not exceed 40% before the first application is made.

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Format updated March 29, 2011