The fruit on the crabapple may develop similar scabby areas and when severe, may cause misshapen fruit that doesn't fully develop. Remove the fallen leaves from the ground, especially in fall. Prune to open up the branching for better air circulation. Avoid getting water on the leaves whenever possible. Minimize stresses to the health of the tree by keeping it well-watered and fertilized.
Fungicides require several applications.
Read the label and follow directions carefully. Nothing has been found to be very effective in treating diseased trees, but since it is not immediately fatal, preventive measures can be taken for the following seasons. Left untreated, it is almost sure death for the tree. A bacterium called Erwinia amylovara. The infection is usually first noticed as a sudden wilting at the tip of a branch.