The most likely causes of the curling are one of the aphids that adore prune/plum trees. But control is difficult. Applications of horticultural oil before leaf bud break can tamp down early infestations but, in some years, they have an amazing capacity to recover. If you look on the underside of the leaves, you should see some of the tiny aphids and probably cast skins of others. Many years, natural predators control the later flushes but usually the first one (about now) is less managed by predators. I think this is an early aphid year in general so am not surprised you are seeing symptoms of their activity. For any spray that is not systemic to work, there must be a real effort to get the treatment under the leaves. This isn't easy. Do you find that, typically, later leaves are normal? If so, that indicates you are getting good natural control. But are they affecting fruit quality? If so, treatment makes some sense although homeowner choices are very slim and often only marginally effective. If not, probably no treatment is needed. Fall treatment is a possibility. Here is a link to the PNW Insect Handbook section on these aphids. Let me know what you think. You can contact me directly with more questions. Chip Bubl  OSU Extension Agent/Columbia County   <personal data hidden>

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