The fungal colonies are olive green to black on mature fruit. Colonies can be round or feathery, have diffuse margins and cover the entire fruit, says Edward Sikora, Extension plant pathologist with the Alabama Cooperative Extension System. The complex of fungi that cause sooty blotch grows on the waxy cuticle of the apple fruit. These fungi survive from one season to the next on apple twigs as well as on other perennial vegetables that have a waxy cuticle. The fungi are spread by wind and in windblown rainwater to developing fruit in the spring and early summer.
Secondary spread occurs from initial sites throughout the summer.
The optimum temperature for growth is between 65 and 80 F, and it usually takes 20 to 25 days after infection for the symptoms to appear on the fruit. Control of sooty blotch is achieved through dormant and summer pruning and tree training that opens trees and facilitates drying of the fruit.