Plum trees can be affected by a variety of diseases common to stone fruit. These are primarily fungal and bacterial in origin, although several viral pathogens also affect plums. Understanding these diseases and how to treat them can help protect plum crops and allow them to thrive for years.
Plum pockets, caused by Taphrina fungi, produce characteristic balloon-like pockets on fruit that can be as much as 10 times the size of the fruit itself. Affected leaves will curl. This disease can cause significant fruit loss.
Spraying in the spring before bud emergence with sulfur-based fungicides will help to reduce the risk of infection.
Black knot is caused by the fungus Dibotryon morbosum. It produces elongated growths or knots on the smaller branches.
Remove all branches with knots. Treat the tree with a sulfur-based fungicide before budding occurs, and withCaptan or a similar agent after.
Bacterial canker (or bacterial spot) is caused by the bacteria Psedomonas syringae, and produces brown areas on the trunk and branches of the affected tree. These areas are water-logged and may exude gum. The leaves can be affected and often develop many small holes.
Remove any infected branches. Carefully cauterize any trunk lesions with a propane torch. Apply zinc and boron supplements in spring and fall to help prevent further infection.
The Verticullum alboatrum fungus can affect plums, causing verticulum wilt. Leaves will wilt quickly and drop, beginning at the base of the affected branch and moving up the branch. Discolored streaks are noticeable when the bark is peeled back.
Currently, no effective treatment for verticulum wilt exists. Removal of the infected tree is required to prevent spreading of the disease.
Brown rot is caused by infection from Monilinia fungi. The blooms will wither and die. Branches around the blooms may also be affected. Fruit will rot. Gray or tan spores will develop and the disease will then spread.
Remove all diseased parts including fruit, blossoms and branches. Change irrigation methods so that water does not come in contact with blossoms or fruit. Apply copper-based fungicides when flowers are just budding to help prevent infection.
These cankers are brown, sunken, irregular areas on the bark of the tree. Yellow gum may also be present. The leaves on an infected branch may wither and die. Occasionally orange, thready growths may be observed on the cankers.
Remove all infected branches well below the infection. Change irrigation methods to lower watering.
Phytophthora root and Crown rot
These illnesses, produced by Phytophtora, are usually caused by improper watering. Infected trees will usually die rapidly at the first sign of warmer weather. Leaves will turn, dark areas will develop around the crown and sap may exude from the bark. The inner bark typically will show reddening.
Good watering practices are important to preventing this disease. Remove soil from around the trunk of the tree down to the roots and remove any infected bark directly above the crown of the tree. Treat trees with a fungicide such as fosetyl-a, to prevent the disease from recurring.
Powdery mildew occurs primarily on the leaves and sometimes the fruits and blooms of plum trees, and is usually caused by either Podospaera or Erysiphe fungi. The name is derived from the white, powdery spores produced by the fungus. Certain types of plum trees are particularly prone to this infection.
Good planting practices with proper pruning for ample light and aeration of the tree are important to preventing infection. Fungicides and agricultural oils can also be used to treat the disease. Biological fungicide can be used on powdery mildew.
Several types of viral diseases affect plums. The most common is Prunus ringspot virus. It primarily affects the leaves of the tree, causing yellow rings and mottled spotting. Eventually it leads to leaf deformity and death and can severely limit the tree's development.
No cures are available for viral diseases of plums. Removal of the tree is essential to prevent further infection within the orchard.