Answer: Crabapples are susceptible to a fungal disease called scab. This disease causes yellow and brown spots on leaves, and can affect the fruit, making it deformed and spotty. Once scab attacks a tree it's difficult to control. The best thing for you to do is to rake up and distroy the fallen leaves and remove the fruit at the end of the season. Then in January or February, spray your crabapple tree with a fungicide (such as lime-sulfur) to prevent the disease from reappearing next year. Lime-sulfur is also marketed as dormant fruit tree spray or Bordeaux. It's used on apple trees as a precautionary measure because apples are quite susceptible to scab, too. Best wishes with your crabapple!
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