My crabapple has begun to drop some of its leaves. Why?

FAQ
Question: 

My crabapple has begun to drop some of its leaves. Why?

Answer: 

The leaf drop is probably due to apple scab. Apple scab is a fungal disease caused by the fungus Venturia inaequalis. Cool, wet weather in spring favors apple scab development. Crabapple varieties differ in their susceptibility to apple scab. Some varieties are very susceptible to the disease, while others are resistant to apple scab.

Apple scab appears as velvety, olive-green to black spots on crabapple leaves. Heavily infected leaves turn yellow and fall from the tree. Highly susceptible crabapple varieties may lose a large percentage of their leaves by mid-summer. Fortunately, apple scab does not kill affected trees. The damage is mainly aesthetic.

Apple scab can be prevented by applying fungicides from bud break through mid-June. For most home gardeners, however, controlling apple scab with fungicides is not practical. Sanitation also plays a role in controlling apple scab. Raking and destroying the leaves as soon as they fall may help reduce the severity of the infection next season. However, the best way to prevent apple scab is to plant scab-resistant crabapple varieties.