Marci Ann: The white stuff you are seeing on your plum tree are the"cast skins" of aphids. This has been an exceptionally good (if you were an aphid) year for them. Usually the leaves get quite sticky from the "honeydew" the aphids secrete from feeding on the leaves. The aphids themselves are small green or sometimes grey or even tan, since several species could be involved. They are usually on the newest foliage, on the underside of the leaves and you may need a magnifying lens to see them. Their honeydew drops to the upper surface of the leaves below. Often, you will see a black "sooty mold" that feeds on the honey dew. None will kill the tree. The numbers seem high because the aphid predators (lady beetles and many others) are much lower than normal. Why that is, we aren't sure but it could be a result of the heat dome last summer or possibly the very extended cold, wet weather we had this spring. You don't need to do anything. It shouldn't be so bad next year. Below is a link to a discussion of the aphids on plums. Hope this helps. Feel free to call or email me directly. Chip Bubl  OSU Extension Columbia County.<personal data hidden> <personal data hidden>

https://pnwhandbooks.org/insect/tree-fruit/plum-prune/plum-prune-aphid