The cool, wet weather this spring has created ideal conditions for the development of apple scab on susceptible crabapple varieties. Scab is caused by the fungus Venturia inaequalis.
Initial symptoms are small olive-green to black spots on the foliage.
Eventually the leaves turn yellow and fall from the tree. Highly susceptible crabapple varieties may lose most of their foliage by midsummer. The heavy defoliation will weaken the trees somewhat, but will usually not kill them. The damage is mainly aesthetic. Heavily defoliated trees are unattractive.