Answer: I think there was a pollination problem with your plum tree. Incomplete pollination will result in malformed fruit. I can't really tell if it was an insect problem when the tree was in flower, or some chemical damage (if you sprayed your tree) or a disease problem of some sort. I can only suggest you take a few of the affected fruits into your local cooperative extension office so trained staff can dissect the fruits and determine the cause. Contact University of California Cooperative Extension, 1045 Arlington Drive, Costa Mesa, CA 92626, phone 714-708-1606. Good luck with your plum tree!
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