Our plum tree has lots of fruit. But in mid to late May the fruit starts to drop while it is the size of an acorn. Very little and won't stay on the tree to harvest.
How to Prevent Plums From Dropping Prematurely
Certified GKH Gardening Expert
There are many reasons this could be happening such as "June fruit drop", the age of your plum tree (it may not be mature enough), a self-thinning process when its heavily loaded, lack of pollination, or diseases/insects. More information on these can be found in the following article: