Scab occurs wherever peaches, nectarines, plums and apricots are grown. The disease affects fruit, leaves and young green twigs. The principle loss from scab is the unsightly spotting of the fruit skin, but premature defoliation and a dieback of infected twigs can also occur. Losses are generally greater on peaches than on nectarines, plums and apricots. Wet conditions during spring and early summer after petal fall are necessary for a severe attack by the scab fungus. The disease is usually more serious in low-lying, moist, shady areas where air movement is poor.

Symptoms:

Symptoms first appear on fruit when they are half formed to nearly full-grown--six to seven weeks after petal fall. Small, round, olive-green spots about l/l6 - l/8 inch in diameter develop on the fruit. Spots are most common near the stem end on the side of the fruit exposed to the sun. Spots are superficial and slowly enlarge. They may merge to form large, irregular blotches that turn velvety, dark olive-green or black. Severely infected fruit may be stunted, become misshapen, or crack open. When fruits crack open, they are vulnerable to invasion by other fungi that can rot the fruit. Scabby fruit may drop prematurely and does not ship or store well. Leaves may also be infected by scab. Small (l/4 inch), round, and yellowish-green to yellowish-brown spots develop on the underside of the leaf. Diseased leaf tissue may dry up and drop, leaving "shot-holes" in the leaves. If the season is wet, scab-infected leaves usually drop early.

Peach scab symptoms on fruit. Photo from the University of Illinois Extension Service.

Causal Organism:

Scab is caused by the fungus Cladosporium carpophilum. The fungus overwinters in twigs infected the previous year. During spring and summer, large numbers of microscopic spores (conidia) are formed on the twig lesions. At 100 percent relative humidity, 20 to 30 hours are required for abundant sporulation in twig lesions. The spores remain firmly attached to the twig until they are moistened. When moistened, they are spread by splashing rain or windblown mist to developing fruit, twigs and leaves. Spores are produced in the greatest numbers about two to six weeks after the shucksplit stage of development (when 1/2 of the developing peach is exposed by the slipping and splitting of the shuck) during warm, wet weather. Spore germination and fungus growth is most rapid at 65-75 degrees F. If weather conditions are favorable, infection begins to occur at about shuckfall. The fruit remain susceptible until harvested. Forty to 70 days elapse from the time the spore lands on the fruit until the disease is visible. The disease is thus not usually observed until the fruit are well grown. Spores from the fruit re-infect the twigs and leaves, completing the disease cycle.

Control:

1. When planting an orchard, avoid low-lying or shaded sites with poor air circulation and soil drainage. Any practice that promotes faster drying of fruit and foliage will help reduce the risk of infection. Destroy nearby wild or neglected peaches, nectarines, plums and apricots.

2. Remove and destroy scab-blighted twigs before growth starts in early spring.

3. Backyard growers should remember that infections are generally superficial. If infection is not severe, fruit quality is minimally affected. Peeling infected fruits make them quite acceptable for processing (canning or freezing) or eating fresh.

Adapted from Michael A. Ellis, Ohio State University Extension, 2000