Peach scab is an important disease of peach and nectarines and can be extremely damaging in warm, humid areas especially in the southeastern United States. The disease appears to affect all cultivars of peach and is known to occur on nectarines, plums and apricots as well.

Symptoms

Fruit symptoms first appear on young fruit as green to olive, circular spots concentrated near the stem end. As these spots enlarge, they deepen in color and become black and velvety in appearance. Fruit lesions are most common on the shoulders of the fruit, but can occur anywhere on the surface. Where numerous, they often coalesce and lead to fruit cracking, allowing rot organisms to enter, especially brown rot.

Twigs and leaves can also be infected but symptoms are more noticeable on fruit. Twig infections occur on new, green, tender growth. Lesions on infected twigs are slightly raised, circular to oval, and become purple to dark brown later in the season.

Disease Cycle

Peach scab is caused by the fungus Cladosporium carpophilum. The pathogen overwinters in small twig lesions on last season's shoots and on the surface of the bark. Spores, produced in these cankers during the early spring, usually around shuck split, are splashed by rain to young fruits and new shoot growth. Rain is required for infection. A very long incubation of 40 to 70 days is needed for symptom development. Although fruit remain susceptible from shuck fall until harvest, usually only infections that occur during the shuck split to pit hardening stage of development have an opportunity to show symptoms before harvest. Twig infections that result in the formation of small overwintering lesions can occur throughout the season. Secondary infections may occur on twigs, but usually do not appear on fruit except on late season cultivars.

Disease Management

Peach scab is controlled by pruning and fungicidal sprays. Pruning helps increase air circulation which aids in the drying of fruit and also allows good spray penetration. Fungicide protection is critical 2 to 6 weeks after shuck split and maintained until 40 days prior to harvest.