Answer: Crabapples are susceptible to a fungal disease called apple scab. The symptoms are what you describe and a preventative spray program should begin in the early spring. You cannot remedy the situation once the disease begins, but you can help stop it from spreading by spraying with a fungicide to protect any plant parts that are not affected at this time. Fungicides for crabapple trees are bordeaux 8-8-100, captan, captan and insecticide combination spray, chlorothalonil (do not eat fruit), propiconazole (do not eat fruit), ferbam, lime sulfur, mancozeb, or thiophanate-methyl. Apply all pesticides according to directions on the label.
Be sure to rake and remove fallen leaves and fruit at the end of the growing season, and start your spray program in the early spring to protect your tree next year.
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