Plum trees are valued for their spring blooms as well as their fruit. They are grown commercially, used as landscape design elements and as fruit or ornamental trees in back yards. They are susceptible to a number of common fungal diseases that affect many plants, as well as a couple that are specific to stone fruit trees, or plums in particular.
Fungus is a living organism. Some can be beneficial, but those that feed on plants, such as plum, trees are detrimental. They attack living tissue, removing the nutrients from it and creating disease within the plant host, just like a parasite. They reproduce by making spores which are spread to other plants by wind, rain, insects and animals, including humans.
Common fungal diseases include verticillium wilt, brown rot, perennial canker and powdery mildew. Fungus diseases more common to plum trees include plum pockets and black knot. These can be transmitted from plum tree to plume tree or from one plant to another, spreading the disease as they go. Some diseases, like powdery mildew, are more of a nuisance that a serious threat, while plum pockets, black knot and verticillium wilt can be more serious, even fatal.
Powdery mildew appears as a whitish or grayish film on leaves before causing a curling of the foliage. Verticillium wilt is a soil-borne fungus that causes leaves to appear pale and dull before falling off the branch. It sometimes affects just one side of the tree at a time. Brown rot affects all parts of the tree, from blossoms to leaves to fruit. Brown flowers cling to the tree well into summer and fruit develops with a brown, rotted appearance. Perennial canker forms oval cankers on branches or the trunk, which enlarge every year. Plum pockets produces plum eight to 10 times the size of the normal fruit which are discolored and misshapen. Black knot forms small, dark knots on younger branches which can elongate to 1 foot in length. They spread from branch to branch, and eventually to the trunk.
Powdery mildew is a mild disease, but left untreated it can weaken the tree, making it susceptible to attack from insects or other diseases. Trees affected by verticillium wilt may be stunted in growth and if the infection is serious enough, can die. Brown rot destroys the entire fruit crop of the tree and because it survives through winter, can do so in subsequent years. Perennial canker can eventually girdle limbs or the trunk, resulting in death. Plum Pockets can also destroy an entire crop and continue in subsequent years. Black knot can also girdle limbs and trunks and bring about death.
Benomyl fungicide can treat powdery mildew with application before and after bud break and can be used as part of the treatment for black knot when applied after bud break through mid-June at two-week intervals. Plum pocket can be treated with dormant sprays before bud break and applications of lime sulfur before buds swell in the fall after leaves drop. Trees affected with black knot also benefit from lime sulfur before bud break and captan after bud break through mid-June. Brown rot, verticillium wilt and perennial canker do not respond to fungicides. Pruning away affected branches with bleach-sterilized tools and removing fallen branches and rotting fruit from the tree and beneath reduce the opportunity for diseases to spread. Do not plant trees in poorly drained soil or near other plants that have been or may become infected with these fungal diseases.