Apple scab is a fungal disease that can be extremely severe, causing leaf loss, reduced tree vigor, and reduced flowering of crabapple and apple trees.
It is the most common and persistent disease affecting these trees. The most common symptom of apple scab is olive-green spots on the leaves. These spots enlarge, darken and become velvety over time. A leaf with several spots eventually turns yellow and drops from the tree prematurely. Heavily infected trees may be defoliated by mid-summer.
The spores of Rhizosphaera are spread most easily by rain splashes. Pruning the tree to increase air circulation will help reduce infections. There are also fungicides that may be applied systemically.