Plum trees are afflicted with pests less often than are most fruit trees. However, there are a few pests that can be truly devastating if given the chance, such as the plum curculio. The plum curculio is a 1/2-inch-long beetle that is common just about everywhere east of California. It also plagues apples, peaches, cherries, pears, and apricots.
Plum Curculio's Life
Adult plum curculios overwinter in the soil and emerge in the spring. Females pierce the developing fruit and lay eggs under the surface. The larvae tunnel deep into the fruit, causing it to rot and drop. The easiest and most effective way to control these voracious insects is to follow a low-toxicity spray schedule recommended by your local county Extension Service office.
Another method offers some control in the case of minor infestations, and can be used to monitor populations to time sprays. Starting just after bloom, check the newly formed fruit for egg-laying scars. Early in the morning, spread plastic sheets under each tree and jar the limbs to check for adult weevils. They'll fall from the tree and play dead, looking like bud scales or pieces of debris. Carry the insects away to their demise before they know what hit them. This procedure must be repeated daily when the bugs are most active in the spring, shortly after the fruit set, and then periodically throughout the summer. As soon as you find adults or egg-laying scars, apply the first insecticide, then spray again 2 weeks later.
Good cultural practices can also reduce plum curculio damage; they will increase the effect of insecticides, but will not provide the same degree of control if used in place of them. Collect and burn wormy fruit. Prune the trees to keep them open, since curculios do the most damage in deep shade. Lightly cultivate the soil around the trees in late spring and early summer to destroy the larvae and pupae in the soil. In late fall, remove all leaf litter and mulch from around the trees; curculios hibernate under it during the winter. Also clean up overgrown hedges and fencerows, two other favorite overwintering locations.
Black Knot Fungus
Black knot and brown rot are fungus diseases that can attack the trees. Remove all blighted twigs and fruit, including what falls to the ground. Remove refuse after the last harvest.
This article is a part of our Fruit Gardening Guide for Plums.