Apple scab, caused by the fungal disease, Venturia inaequalis is one of the most serious and disfiguring diseases of ornamental and fruit apples. Although the name implies it is limited to apples, this disease can be found on many members of the rose family. The hosts can include apples, hawthorns, fire thorns, cotoneasters, and pears.
The symptoms of this disease are typically olive brown lesions on the leaves and sometimes on new succulent shoots. The scabs are indistinct at the margins and if the infection is severe they may coalesce into larger patches. The patchy lesions can be found on flowers, fruit, leaves and twigs. The tree responds to this infection by prematurely dropping infected leaves. Repeated severe infections can cause die back after several years.
The disease overwinters in dropped leaves and affected twigs remaining on the tree. Wet spring conditions spread spores and allow them to germinate on the leaves, initiating new infections. The warmer the temperature, the shorter the amount of time it takes to initiate and infection. The peak period of infection occurs when the trees are starting to flower and continues until full bloom. Secondary infections will occur throughout the spring and summer when conditions are wet for long periods of time.
Some cultural control, including disposal of diseased leaves and twigs away from infected trees as well as the removal of all leaves in the fall will help to reduce the intensity of future infections. Planting disease resistant varieties and avoiding mass plantings also reduce the amount of disease inoculum and the potential for sever outbreaks.
There are a number of broad spectrum fungicides that are labeled for control of apple scab. Typically they require several applications starting at bud break and continuing every two weeks to get suitable control.