Bacterial spot and black knot are two diseases that affect plum trees. Bacterial spot affects the fruit, leaves and twigs and can cause significant fruit loss. Black knot causes stunted growth in trees as well as dark-colored knots on the limbs.
Another disease, brown rot, also affects plums as well as peaches, apricots, nectarines and cherries. It's caused by the fungus Monilinia fructicola. An additional disease affecting plum trees is crown gall, which is caused by the bacterium Agrobacterium tumefaciens and is particularly prevalent in fruit tree nurseries.
Cytospora canker is a destructive disease that affects plum trees as well as other fruit trees. It often kills trees in newer orchards, and in older orchards, infected trees become less productive. Plum leaf spot, caused by the fungus Coccomyces prunophorae, can also damage plum trees by harming their leaves.
In the Northeastern United States, the most significant disease affecting plum trees is plum pockets, which are caused by Taphrina communis. Powdery mildew of plum, caused by Podosphaeria oxyacanthae, is also cause for concern for those with plum trees.
Finally, rhizopus rot and plum pox virus can both affect plum trees. Rhizopus rot can occur in fruit on trees damaged by hail, although it's more common after harvest.