Plum trees serve a dual purpose in the landscape; they are grown both as ornamental trees for their spring blooms and reddish-purple foliage, and also for their fresh fruit. The trees have a mature height of 15 to 20 feet with an equal spread and an upright, rounded crown. The presence of sap dripping from the branches indicates the presence of pests on plum trees.
Plum trees are susceptible to a variety of aphids, including the leafcurl plum aphid, hop aphid, rusty plum aphid, mealy plum aphid and thistle aphid. In addition to plum trees, the pests are also likely to infest apricot, cherry and peach trees. The mealy plum aphid does not infest Japanese hybrid plum varieties.
Aphids are small, soft bodied, oval shaped pests that occur in different colors, depending on the variety. They may be winged or wingless. The hop aphid is light to dark green in color, and the female has wings. Leafcurl plum aphids are dark green to red in color and their nymphs are light green. The rusty plum aphid is brown to dark purple and has white stripes on the legs. The mealy plum aphid has a light blue-green color with a powdery coating on the body. The thistle aphid has black marks on its shiny green body.
The females lay their egg in the bark crevices and the eggs hatch just prior to bud break in spring. Aphids have long sucking mouths that they use to suck plant sap from tender areas like foliage, buds and twigs. The pests are found in large numbers on foliage undersides. As the insects feed, they produce heavy amounts of sticky honeydew. This is the sap that is seen dripping from the tree. The sap covers the foliage and fruit and encourages the growth of sooty mold, a black fungus, on infested areas. Aphid damage leads to distorted, damaged and yellowing foliage and shoots.
Introduce natural aphid predators in trees to control the pests. These include syrphid fly, parasitic wasps, lady beetle and lacewing. Prune and destroy the infested areas in the case of light infestation. Avoid excessive use of nitrogen fertilizer as this makes trees more prone to infestation. Chemical control options include the use of pesticides containing acephate, malathion or permethrin.