Alternaria leaf blotch is most likely to occur on 'Delicious' strains and should not be confused with frogeye leaf spot, captan spot, or with'Golden Delicious' necrotic leaf blotch. Frogeye leaf spot usually appears earlier in the season and is associated with nearby dead wood or fruit mummies. Golden Delicious' necrotic leaf blotch commonly occurs in July and August as a result of physiological stress caused by fluctuating soil moisture. Typically, the incidence of fruit infection is relatively low, but in heavily defoliated orchards, fruit infection as high as 60 percent has been reported. The fungus can overwinter as mycelium on dead leaves on the orchard floor, in mechanical injuries in twigs, or in dormant buds.
Primary infection takes place about one month after petal fall.
At optimum temperatures infection occurs with 5.5 hours of wetting, and lesions can appear in the orchard two days after infection, causing a serious outbreak. The fungus produces a chemical toxin which increases the severity of the disease on susceptible cultivars. Determine percent leaves infected in areas of the orchard where mite injury is most intense. Continue to monitor mites to maintain populations at less than six to eight per leaf where this disease is a problem. Currently registered fungicides do not provide satisfactory control under severe disease pressure.