Plum trees are susceptible to quite a few pests and diseases, and require regular maintenance. Fertilizing the fruit trees and spraying them with fungicides and pesticides is necessary to promote strong, vigorous growth. Some of the most common plum tree diseases that are caused by fungal infections are black knot, brown rot, powdery mildew, verticillium wilt, perennial canker and plum pockets. The bacterial diseases affecting plums are crown gall, bacterial spot and shot-hole. Plum tree diseases like plum pox virus and plum rust are caused by pests like aphids.
Black knot, which is caused by the Dibotryon morbosum fungus, causes long, hard, black knots to appear on the plum tree branches and sometimes on the main trunk. Perennial canker is a similar fungal infection caused by the Valsa cincta and V. leucostoma fungi, and causes oval cankers to appear on the branches and trunk. In both cases, the knots and cankers eventually girdle the tree and kill it.
The brown rot fungus grows in the form of brown tufts on plum blossoms, fruits and small branches, and causes the flowers to wither and the fruits to rot. A lot of fruit destruction is also wrecked by the taphrina fungus, which creates hollow, enlarged spaces known as plum pockets in the plum fruits. The Podosphaera oxyacanthae fungus causes powdery mildew, a white powdery growth on the underside of the plum leaves that destroys the leaves and stunts the tree growth. Verticillium wilt, which is caused by the Verticillium alboatrum fungus, also stunts plant growth by causing leaves to wilt, the wood to discolor and the roots to rot.
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The crown gall infection, which is caused by the Agrobacterium tumefaciens bacteria, produces corky tumors in the plant tissue. The other two bacterial plum tree diseases, bacterial spot and shot-hole, are caused by the Xanthomonas pruni and Pseudomonas syringae bacteria. Spots and holes appear on the plum leaves, and the leaves then turn yellow and drop off. The bacterial infection also produces holes on developing fruits, causing them to rot. Aphids spread the plum pox virus and plum rust. In the first case, fruits production is affected with the developing plums deforming or rotting, and, in the second case, the plum leaves turn silver, curl up and wither away.
Fungal infections are wind-borne or soil-borne, while bacterial and viral infections may be soil-borne or carried by pests. To mitigate plum tree diseases, it will help to avoid planting a plum tree in a previously affected area. When caring for a plum tree that is already infected, it will be necessary to prune and burn the diseased parts, and to spray the tree with fungicides, bactericides and pesticides. It is important to note that fungicides work mainly as preventives, and may not have much effect on severely infected trees.