December 2014 Plant Disease Update
At this time of year after leaves have fallen, black galls or “knots” may be evident on the branches of plum or cherry trees. These galls are covered with the fungal fruiting structures of a common pathogen that infects many Prunus species, but is most common on cherry and plum trees. This disease is aptly called “black knot”. In the first year after infection, galls are greenish and develop slowly and may not be very noticeable, but during the following growing season they enlarge dramatically and turn black as fungal fruiting bodies form on the surface.
Galls expand lengthwise annually and can expand to about a foot in length over time. The fruiting bodies release spores for new infections in the spring, so galled branches should be pruned out 4” below the galls in the winter or as soon as they are noticed. Pruned branches should be buried, burned or removed from the location. Severely galled branches can result in branch loss and, when numerous, eventually kill the tree. Preventative fungicides sprays to protect new growth in the spring can help control this disease, but only if pruning is also practiced. Note that fungicide products labeled for control of this disease on ornamental trees differ from those labeled for use on fruit-producing trees and be careful to follow label directions.
Wild cherry and plum are very susceptible to black knot and often serve as an inoculum source for fruit-producing or ornamental plum or cherry trees growing near wooded areas. When planting plum trees, select black knot-resistant varieties when possible. No cherry trees with reliable resistance to the disease are available. If you have plum or cherry trees and haven’t taken a good look at them since the leaves have dropped, now is a good time to check carefully for the presence of galls and prune any galled branches.