Q. What is the black foam/fungus on the twigs of my plum tree?
Henrietta from Youngstown
A. This disease has a name that matches its appearance: black knot. It is a destructive disease of both cherries and plums. It is seen on plums much more than cherries.
Infections usually appear on the newest growth first, starting as foamlike, expanded deformities on these branches. As the disease progresses, these areas expand and rupture. The color of these areas then turns black and hardens with time, according to our fruit pathologist, Dr. Mike Ellis.
The disease may start on just a branch or two, but it will spread to several branches on the tree if left untended. Backyard fruit growers often miss the signs of infection until much of the tree has black growths on the branches. As the disease spreads on the affected tree, it is also spread to other trees in the neighborhood.
So what can be done? First, when planting a plum tree, consider choosing a variety that is less susceptible to the disease. (A list is on the fact sheet by Dr. Ellis below.) Next, be observant in the home orchard. On the first nice day in March, backyard fruit growers should check trees for disease. Affected branches should be pruned 2-4 inches below the infection. These branches should be destroyed and removed from the area, not composted. Heavily infected trees can be pruned back, but in some cases may need to be taken out completely and replaced by another species of fruit tree other than plum or cherry.
Fungicides are another option for controlling this disease. Timing in early spring is critical. An OSU Bulletin with information on spray schedules can be obtained through the OSU Extension office in Canfield. In addition to this publication, we have the Midwest Home Fruit Production Guide to assist home gardeners with fruit production. This complete book has color pictures and descriptions for growing all fruit crops in the home garden.
For more information from Dr. Ellis on black knot, go to: go.osu.edu/blackknot.
Eric Barrett is OSU Ext. educator for agriculture and natural resources in Mahoning County. Call the office hotline at 330-533-5538 from 9 a.m. to noon Mondays and Thursdays to submit your questions.