Certified GKH Gardening Expert
Premature fruit drop is normally the result of poor pollination or an insect problem. Peach tree borers can also affect plums. This article will help determine if this is a problem: https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-borer.htm This article may help as well: https://www.gardeningknowhow.com/edible/fruits/plum/plum-trees-not-fruiting.htm Overbearing is another possibility. When the fruit is pea size, begin thinning to prevent fruit drop. This will reduce the energy of the tree so it can put more into the production of fruit. If pollination/fertilization is at too great a level for the tree to sustain, all of the set fruit will simply drop. You can also thin the blossoms rather than the fruit.