Plum trees are susceptible to the same diseases that most other stone fruits suffer from. Viruses, bacteria and fungi infect plum trees. Recognizing the type of disease will help you determine how to control it. Symptoms in each category are similar and sometimes indistinguishable, especially to a novice gardener. If you are unsure what ails your tree, take a sample to your local county extension office for diagnosis and advice.
The best prevention for disease in plum trees is a healthy plant. Protect trees from injury and mechanical damage due to lawn mowing and improper pruning. Disease often enters the tree at the point of injury. Fertilize with a balanced formula only when needed. Avoid over fertilizing. Prune away diseased or damaged branches and protect the cuts with sealing compound. Commercially propagated plum trees are grafted onto resistant rootstocks. Choose a tree with rootstock that is resistant to the most common problems in your area.
Control fungal diseases by using a fungicide spray combined with proper pruning, including dressing the cut with a sealing compound. Remove damaged and infected branches before the buds appear in the spring, cutting at least 4 inches below the diseased wood.
Bacterial diseases are not easily controlled. Bacteria is spread through the soil and by wind and rain. Avoid planting susceptible trees near infected plants and allow several years before replanting in the same spot.
Viruses infect the tree through contact with diseased plants, through diseased seeds and by grafting with diseased tissue. The only control is to remove infected trees and use disease free trees from a reputable nursery.
Fungal diseases are most active in the spring during flowering and after the fruits have started to ripen. Often, small cankers or growths will appear on the branches, eventually strangling the branch. The growths are the source of spores that spread the disease.
Black knot is a common disease in plum trees caused by the fungus Dibotryon morbosum. Soft green knots appear, eventually becoming hard and black.
Plum pockets are caused by a Taphrina fungus. The fruit is distorted in size and appearance, forming a hollow fruit that is 8 to 10 times its normal size. Some Taphrina fungi also deform the tree, injuring or killing the shoots.
Brown rot, caused by the Monilinia fructicola fungus, also attacks plum trees. Often the first sign of brown rot is infected blossoms that turn brown and remain on the tree through the summer or ripe fruit that develops a brown rot. The branches also become infected, forming a canker which can eventually strangle the branch. Young shoots wither away.
Perennial cankers are slow growing, oval cankers that grow until the branch or trunk is girdled and dies.
Verticillium wilt is caused by a soil-borne fungus. The leaves are light colored and lack luster, eventually dropping off the tree. The fungus invades the plum tree through the root system and spreads up the tree. Plum trees on resistant root stock are available.
Powdery mildew is a fugal disease that infects plum. A powdery white fungal growth covers the leaves. Black spots eventually form on the white surface. The leaves may curl up and shoots may be distorted.
Plum trees are also susceptible to bacterial diseases such as bacterial spot and “shothole”. Infections by these diseases are usually first seen on the leaves. Leaf spots occur o the under side of the leaf, eventually turning black. The dead leaf material falls away, leaving holes in the leaves. Eventually the leaves turn yellow and drop off. Infected fruits have brow spots. Twig infections appear as cankers in the spring.
Crown gall often infects the tree at the crown, but can also occur on the roots or higher on the stem. The bacterial infects the tree through an injury. Pruning may worsen the infection.
Plums are susceptible to numerous virus diseases. Viruses rarely kill the tree, but they may weaken the tree and leave it vulnerable to injury and other diseases.
Prunus ringspot is one of the most common viral diseases, causing leaf abnormalities and stunting tree growth. Other viruses have similar symptoms.