Plum trees, both ornamental and fruit bearing, are susceptible to fungal and viral diseases. The damage can range from misshapen leaves to stunted growth to loss of fruit and even the death of the entire tree. Any sign of abnormality in the appearance of the tree should be looked into and taken care of immediately to lessen the chance of permanent damage and/or the further spread of the disease.
Black knot is a serious fungal disease that occurs in a large part of the growing area. It will appear first as soft green knots or swellings which turn to hard, long, black cylindrical galls on small branches. In most cases, the branches will die. If the infected branches are not removed immediately, the disease can spread and the tree will become stunted and eventually die. Prune to at least 4 inches beyond any sign of infection. The pruning tool has to be sanitized between each cut to prevent spreading the disease. Fertilize well to help the tree recover and spray with a fungicide the next spring.
Plum Pox Virus
Plum pox virus is spread by an insect, the aphid. It attacks fruit-bearing plum trees. It will appear as brown or yellow rings or blotches on the fruit itself, which may become deformed. The symptoms can appear sporadically, only at the base and not the tip of the branch or only on some of the fruit. It can take up to 3 years from the first infection for the symptoms to appear. While it will not kill the tree, it will affect both the quality and quantity of the fruit and eventually the tree will not produce any fruit at all. Using insecticides can help with the control of aphids, but it is almost impossible to eradicate them completely.
Brown rot is a fungal disease that attacks the flowers, small branches and the fruit itself. Flowers turn brown, wither and die. Fruits develop soft brown spots that grow and become covered with tan-colored spores. The fruit rots away, shrinks and dries out. The twigs are infected if the flowers do not fall off. The infection spreads through the flower stem and the twigs develop cankers. Leaves on the infected twigs turn brown and wither. The disease winters over in the fruit, twigs or cankers. Weather conditions contribute to the development of the disease when the flowers are wet for 5 hours or more and the temperatures are between 60 and 70 degrees F. It can also happen at any temperature if the flowers stay wet for longer periods of time.