In spring, leaves of flowering crabapples develop scab disease, caused by the fungus Venturia inaequalis. This is a common fungal disease on susceptible varieties of crabapple and is especially active during wet spring weather. When leaves become infected with scab, they develop olive-green to black spots on the leaf. Infected leaves often turn yellow and drop from the tree, resulting in premature defoliation.
Although fungicides are available to prevent scab disease, frequent, timely applications are needed. Raking and removing fallen leaves helps to reduce infection the following spring. The best defense is to plant a crabapple cultivar resistant to scab. Contact your local Extension office for fungicide and cultivar recommendations.