black spots and yellow leaves on crab apple tree
It sounds like you've got apple scab, a common fungal disease of crab apples and apples. Apple scab gets off to a great start with humid spring weather. Once it has spread, it's pretty futile to start spraying a fungicide. Rake up and remove all fallen leaves and fruit so that next year there won't be as much spore acitivity. Then start spraying a fungicide as soon as you see green tips on the buds next spring. You'll have to spray repeatedly, about every 7-10 days for two months. The good news is newer crab apple trees are bred for their resistance to diseases like apple scab.
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