Black Growths on Plum Tree
There are several black lumpy growths on the stems of my plum tree. Will this kill my tree? How can I keep it from spreading to my other plants?
The black growths on the stems are caused by black knot fungal disease. This disease attacks all Prunus including cherries, ornamental plums, peaches and other stone fruits. Fortunately it will not attack other types of trees. The stem swellings can girdle (stop flow of water and nutrients) limbs and stunt the plant's growth. Prune out and destroy the green knots in spring to reduce the risk of new infections. Unfortunately there may be other disease-infected plants in nearby natural and cultivated landscapes. These serve as sources for future infection for your and other nearby susceptible plants. There is some work on biological controls so there may be some good news in the near future. Look for disease resistant varieties whenever possible. President plum is highly resistant while Methley, Milton, Early Italian, Brodshaw, Fellenberg, Shiro, Santa Rose, and Formosa varieties are moderately resistant.