Plum trees are an excellent choice for home gardeners who wish to grow delicious fruit. While plum trees are fairly hardy, they are susceptible to insect infestations and diseases that can affect the health of trees and the quality of the fruit they bear. Proper identification of insects and diseases is key to keeping damage to a minimum and growing juicy plums.
Insect infestations can cause significant damage to plum trees and cause the tree to produce low quality fruit. Scale insects are common pests of many trees, including the plum tree. These pests cause damage to the leaves of the plum tree by extracting plant sap with sharp mouths. Scale insects have a waxy coating that protects them from predators and insecticides. Plum curculio is a damaging pest of plum trees and is native to North America. The adult form of the plum curculio is a snout beetle that measures 1/4 inches in length and is dark brown in colour. The larvae of this pest is a white grub that also damages plums.
Scale insects cause plum trees to develop tiny yellow lesions on foliage, distorted stems, reduced vigour and branch dieback. Plum trees infested with scale insects may also lose their leaves early and may eventually die if infestations are heavy. Plum curculio damages plum trees in several ways. The adult beetle feeds on buds, flowers and new fruit for six weeks during the spring. Beetle feeding may cause cat facing on plums. The adult beetle places her eggs inside the fruit, using crescent shaped slits. The eggs mature within five days and the larvae hatches inside the fruit, where the larvae feed until they mature.
Brown rot is a serious disease of plum trees, as well as many other stone fruits. This disease is caused by the fungus Monilinia fructicola. Brown rot is least active during the hot summer months, but becomes active during the fall as fruits mature. Although brown rot can affect green fruit, it is most damaging in mature fruit. Another important disease of plums is black knot, which causes black swellings to develop on plum and cherry trees. Wet weather is favourable for the development of black knot, which is most active when temperatures range from 12.7 to 25.0 degrees C. Black knot is a fungal disease that often requires chemical control methods.
Brown rot invades spurs, shoots and blossoms of plum trees. The damage from this disease however, may go unnoticed until fruit begins to turn brown and rot. Cankers may also develop on the trunk of the tree, and young shoots begin to die. Green fruit may develop brown circular lesions, and mature fruit can turn brown and die within hours. Plums infected with brown rot may fall from the tree or remain mummified on the branches. Black knot is characterised by long, black swellings on the branches of infected plum trees. Swellings are usually green in colour and turn black over time. Once tree stems are girdled by these swellings, they die. Plum trees become weak over time and eventually die from black knot.