Problem: Scales (Plum)
Several different types of scales infest plum trees. They lay their eggs on leaves or bark, and in spring to midsummer the young scales, called crawlers, move about and then settle on leaves and twigs. The small (1/10 inch), soft-bodied young feed by inserting their mouthparts and sucking sap from the plant. The legs usually atrophy, and a hard crusty or waxy shell develops over the body. Mature female scales bear live young or lay their eggs underneath their shell. Some species of scales are unable to digest fully all the sugar in the plant sap, and they excrete the excess in a fluid called honeydew, which often drops onto the leaves below. A sooty mold fungus may develop on the honeydew, causing the plum leaves to appear black and dirty.
Apply an insecticide labeled for this pest, following label directions.