There are a number of reasons for plum leaves to curl. One of them is simply dehydration, and can easily be addressed by frequent, deep watering. But when plum leaves are turning colour, curling or dropping in spring or summer, it can be a sign of a serious problem. There are many diseases of plum trees that affect leaves, blossoms, roots and bark. Only a few of these viruses, fungi and insects cause plum leaf curl.
Plum leaf curl aphid
Unroll a curled plum leaf with your fingers. If you see tiny, greenish-yellow, soft-bodied insects, you've got plum leaf curling aphid. Aphids eat the leaf margins, causing them to curl up and eventually drop. They don't stop fruit production, but can eventually damage the vitality of the tree if the infestation is large. Control plum leaf curl aphid with a strong stream of water, rather than chemical applications that also kill beneficial insects.
Look for a greyish-black, slimy film on curling plum leaves, especially on the leaf margins. This fungus is called sooty mould, and it feeds on sticky excretions, or "honeydew" secreted by aphids. Sooty mould won't invade the leaf tissue, but will block some light from its surface. Sooty mould can easily be controlled if aphid populations are addressed with hard spraying with water or insecticidal soaps.
Prunus stem pitting
Look for early, smaller fruit among your plum tree's curling and yellowing leaves. If these symptoms occur in late summer, it's the tomato ringspot virus, which also attacks peach, apricot and cherry trees. The virus attacks when the tree has been partially girdled, and enters through wounded trunk bark. Tomato ringspot is communicated from broadleaved weeds like dandelion, and from other infected trees. It usually defoliates and kills the tree within a few years.