Hi Cathleen,
Thank you for your question about your plum tree curling with aphids. Aphids can be a very frustrating problem to manage.

Amy Jo Detweiler, a member of the OSU Extension Horticultural Staff, says “Generally, aphids on plum trees do not kill them outright but are just an indicator that the tree is under environmental stress. If it is not fully leaving out that suggests that something is interfering with the flow of water and nutrients to the leaf tissue, either a problem with the vascular system of the plant or root system. How does the bark on the branches or trunk look? Any discoloration, cracking, wounds, oozing?”

If you see issues other an aphid damage, I recommend you call Amy Jo at the OSU Extension office in Redmond at<personal data hidden>, ex 79510. Amy Jo welcomes your home garden questions and is a great resource.

A healthy plant will defend itself better than a stressed plant. Create healthy soil, fertilize only when needed, water your trees deeply several times a month from June until September. Protecting your tree is the goal, not necessarily killing all the pests. You may find this article link helpful: Fruit tree integrated pest management | OSU Extension Service (oregonstate.edu)

Specific to aphids and plum trees, following is information about aphid species that attack plum trees and management methods:

“Several aphid species attack plums and prunes, including the leafcurl plum aphid, hop aphid, and thistle aphid. The leafcurl plum aphid varies considerably in color from shiny green to brownish-green or brownish-yellow. Hop aphid ranges from green as wingless adults to near black as winged adults. The thistle aphid is relatively large, shiny green with dark patches on its back. Initially, aphids feed on the shoot tips, which on young trees can cause stunting and malformation of the shoot tips. On older trees, fruit set is reduced in subsequent years if aphid populations are high. Aphids also produce a sticky, sweet honeydew that promotes sooty mold problems on fruit.

Management-biological control
Aphids have many natural enemies, which include lady beetles, syrphid fly larvae, and green lacewings. Avoid broad-spectrum insecticide applications that disrupt the life cycles of these natural enemies.

Management-cultural control
Home orchardists: Wash aphids from plants with a strong stream of water or by hand wiping. Aphid populations tend to be higher in plants overly fertilized with nitrogen. Avoid excessive watering which, together with nitrogen applications, produces flushes of succulent growth.”

Management-chemical control: HOME USE
Chemical controls are provided in the following article. Please note those that are toxic to bees.
Click this link: Plum and prune-Aphid | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)