Insect damage could be causing the problem.
A Drooping leaves followed by defoliation in the middle of the growing season is never a good sign. Lack of water, overwatering to the point where root injury occurs, root rots and wilt diseases Estimated Reading Time: 3 mins. Too much or not enough water can cause wilting and leaves to drop. Even established trees suffer during extremely hot and droughty summers.
Gardeners often overcompensate with improper watering. Both can cause the symptoms you described. The affected tree may be in an area that stays wet longer or dries faster than the healthy plum tree. The tree uses a lot of energy to produce buds, flowers and fruit and will result in leaf drop to compensate for lack of energy.
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When this happens the tree, usually at some point, should begin to put out some new foliage. The new foliage usually appear on the outer branches near the tips.
Hope this helps you!