Peach Disease - Scab

The fungus can be extremely damaging to trees throughout the mid-Atlantic region because of the typically warm, wet weather during the day through the mid-season period. The disease appears to affect all cultivars of peach and is known to occur on nectarines and apricots as well.

Symptoms

The most notable symptoms of peach scab occur on the fruit, where small, greenish, circular spots gradually enlarge and deepen in color to black as spore production begins. Lesions are most noticeable on the stem end of the fruit where spores wash from infected areas of the twigs onto the fruit. Unlike bacterial spot, there is no pitting of the skin and no symptoms on leaves. The overwintering twig lesions are clearly visible during the early season as small, grayish, more or less circular, slightly sunken lesions on the previous season's shoot growth.

Disease Cycle

The pathogen overwinters in small twig lesions on last season's shoots. Conidiospores, produced in these cankers during the early spring, are splashed by rain to young fruits and new shoot growth. Rain is required for infection and a very long incubation of 42 - 77 days is needed for symptom development. Although the fruits remain susceptible through harvest, it is usually only infections that occur during the shuck split to pit hardening stage of development that have an opportunity to show symptoms 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.

Management

Monitoring orchards for peach scab during the current season is an important step for managing the disease the following year.

The critical time for effective disease control begins at the shuck split stage of fruit development. A standard program includes chlorothalonil at shuck split followed by either a captan or sulfur cover spray program to control peach scab. By the time the disease appears, it is too late to do anything about it during the current growing season. Proper and regular pruning facilitates air movement, reduces length of wet periods, and improves spray penetration into trees. Fungicide sprays should be applied at 10- to 14-day intervals and continued until 40 days before harvest. Fungicides used to manage peach scab will also manage brown rot. Consequently, rotating fungicides by FRAC group is encouraged to limit fungicide resistance.