Answer: It sounds like insect feeding, but if you cannot see the insect pests they are very sneaky or very small. Neem oil sprays in the early season, just as the leaves are starting to emerge should provide some protection. Or you can take a cluster of affected leaves to your local cooperative extension office for positive diagnosis. Their plant clinic staff will be able to look at the leaves through a microscope and detect frass (bug poop) which will help identify the problem. Contact: Ottawa County MSU Extension, 12220 Fillmore Street Suite 122, West Olive MI, 49460, Phone: 616-994-4580. Best wishes with your plum trees.
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