The cool, wet weather we have experienced this spring has created ideal conditions for the development of apple scab on susceptible crabapple and apple trees.  Apple scab is caused by the fungus Venturia inaequalis. Symptoms appear in early spring as nearly circular, velvety, olive-green spots on both the upper and lower sides of the leaves. The spots eventually turn dark-green to brown and develop a rough texture (See below).  Affected leaves will turn yellow and fall from the tree. Heavily infected trees may lose nearly all of their leaves by early summer. Fruit develop black, rounded, rough-textured spots.
 
Despite its unsightly appearance, scab does not kill apples or crabapples. In fact, it is a minor problem unless you are growing apples commercially. Most crabapple trees will replace the fallen leaves with new ones within a month or so, and there will be minimal damage to the tree’s health.     
 
The best defense against apple scab is to plant varieties of crabapple and apple that are highly resistant to scab. These varieties will greatly reduce scab problems, even during a rainy spring like the one we have had in 2010. Apple scab may also be prevented through application of several fungicide sprays during springtime, but this option is used mainly by backyard and commercial apple growers to protect scab-susceptible varieties.