Answer: Most likely your crabapple has apple scab which is a common disease of older varieties of crabapples. It is caused by a fungus and is most severe following a wet spring. Since the fungus lives through the winter in diseased leaf debris, it is important to clear away all fallen leaves and fruit from under the tree. This may not totally prevent disease in the future, but it will help lessen it's occurrence.
Chemical control includes fungicides which must be applied before the first sign of the disease occurs. It is critical to follow directions exactly as to WHEN to apply, and how OFTEN. Fungicides used for apple scab control are benomyl and captan. REMEMBER to read the container label and use strictly according to label directions!
When time comes to replace the crabapple, be sure to look for the newer disease resistant varieties. A lot of newer varieties had done away with some of the nastier habits of crabapples, as well as increase their bloom, and resistance to disease. Your local extension office (313-833-3412) should be able to supply you with a list of recommended varieties for your area.
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