|I moved into a house in Coronado, California that has what someone said was a plum tree. It's been 5 years and I haven't seen any plums so I doubt that's it. The leaves, in early summer, start to show signs of damage. It starts with a black slug looking thing, about 1/8
|It is possible that you have an ornamental plum (they flower but don't bear fruit). What you describe sounds like a pear slug. These pests are the larvae of a sawfly and resemble small, black, elongate slugs, hence the common name pear slug. There are two generations a year. Larvae feed on the upper surface of stone fruit tree leaves (including ornamental plum). High populations can defoliate trees.
I think if you control these pests, your tree will regain its health, and you'll be able to grow plants beneath your tree.
Pear slugs pass the winter underground in the pupal stage. During late spring, the adult insects emerge to mate and lay eggs. Adult pear slugs are dark colored, nonstinging wasps approximately 1/4 inch long. They lay eggs in slits made in the leaf.
Young pear slugs feed on leaves and become full-grown in approximately three weeks. Pear slug larvae feed on the upper leaf surface. They avoid the larger leaf veins and rarely penetrate the lower leaf surface. The resultant "skeletonized" leaves have a characteristic appearance. Chewed areas of the leaf turn brown. When heavily damaged, the entire leaf falls prematurely.
Pear slug injury occurs in two peaks during the growing season. This injury coincides with the presence of full grown larvae, which do most of the feeding. In your growing region, damage by the first generation often peaks in late July. The second generation of larvae are usually most active in September. This later generation is often the most damaging and can completely defoliate susceptible plants.
Most insecticides easily control pear slugs when used at labeled rates listed for control of caterpillars or leaf beetles on trees and shrubs. Among the effective insecticides are carbaryl (Sevin, Sevimol), malathion and Spinosad. Pear slugs often can be controlled using a forceful jet of water to dislodge them. Insecticidal Soaps have also been effective for controlling pear slugs. You might also be careful to rake up any fallen leaves at the end of the season, and to mulch over the soil surface with a few inches of organic matter (to keep the adults from emerging).
Best wishes with your garden!