If you have had a leaf disease on your crab apple this year and it does not quite fit the description of apple scab, it may be frogeye leaf spot. This disease is not new, but it does not get as much press as scab on crab apples. Frogeye is the leaf spot phase of a more notorious disease of apple and crab apple called black rot. Limb cankers and fruit rot phases can be quite damaging to apples, but the leaf spot disease causes early defoliation as we see with scab.
Frogeye leaf spot is a fungal disease that has been prevalent in the Midwest the last several years on apple and crab apple. The cause is Botryrosphaeria obtusa. Classic symptoms include numerous small spots on older cluster leaves with marked zones of lighter and darker tissues within the spots. Affected leaves are always near dead or dying limbs that have a black rot canker on them. This same canker and, in some cases, infected leaves will produce inoculum, which then infects developing fruit from August through September. Removing and destroying all dead wood during annual pruning is a highly recommended control measure. Also remove any mummified fruit on the ground or in the tree. Because Botryosphaeria is a canker-producing fungus, you should follow good horticultural practices to improve tree health as you would with any canker disease. This means removing dead wood, watering the tree in periods of drought that last two weeks, and fertilizing in the fall or early spring. Producers of edible apples should use a fungicide program to control the leaf spot phase of this disease. The same fungicides used to control scab should also control the leaf spot on crab apple trees. Report on Plant Disease No. 814 discusses black rot and frogeye leaf spot of edible apple.