Botryosphaeria ribis Gross. & Dug.
Occurrence and importance
Botryosphaeria rot, sometimes called bot rot or white rot, is becoming increasingly important as a disease of apples. It has been reported in eastern, midwestern, and southern States on such varieties as Delicious, Starking, Grimes Golden, Rome Beauty, Stayman, Golden Delicious, Wealthy, and Northwestern Greening. Because at certain stages the disease can very easily be mistaken for black rot, it is probably more common than is generally thought.
Fruit infections may occur throughout the summer, but symptoms do not usually appear until the fruits approach maturity. The first signs of the disease are small, brown specks or spots, usually with red halos (top photo). The spots are often slightly sunken and are 1/8 to 1/4 inch in diameter. They commonly occur at insect injuries, slight skin breaks, and open lenticels. Infection in mature and ripening fruits may develop rapidly under warm conditions, and the entire fruit may become decayed. Badly decayed fruits are typically soft with a somewhat bleached or cooked appearance (bottom photo), hence the name "white rot." Inhibition of fungus growth by cold-storage temperatures prevents the enlargement of rot lesions, but active decay can develop after the fruits have been removed from the cold. Symptoms of botryosphaeria rot in the speck stage and in stages where moderate-sized lesions remain brown and firm are indistinguishable from similar stages of black rot. The variability in symptoms of botryosphaeria rot may be related to differences in environmental conditions during the development of the rot or to inherent differences in strains of the fungus.
Botryosphaeria ribis, the causal agent of the disease, overwinters in cankers or in mummified fruits. Woody tissues and fruits are attacked, but not leaves. A skin break or injury is usually necessary for infection of the fruits, which occurs during rainy periods. Decays initiated through the calyx have been observed on Delicious apples. Ripe fruits are most susceptible to the disease, which progresses rapidly at temperatures above 50 °F. The disease will not progress at cold storage temperatures, but the fungus in incipient infections is not necessarily killed and rots may develop after the fruits are removed from the cold.
The principal control measure is orchard spraying. For best results follow the recommendations of the State agricultural experiment station or extension service.
Botryosphaeria rot, early stages
Botryosphaeria rot, advanced stages