Black knot of wild cherry and plum trees can easily be seen this time of year without leaves on the trees. The fungal disease manifests into large, black distorted growths called galls on the branches. The galls start on young, green tissue when an airborne spore lands on the newest branch tissue. A green swelling will enlarge the branch as the parasitic fungus develops inside, malforming the branch. By summer’s end, the swelling turns black, and can eventually girdle and kill any new growth beyond the site of infection. Black galls can enlarge and grow over several years. Young trees can be killed in a few years, while older trees can survive as long as some branches are not affected.
Black knot is caused by the fungus Apiosporina morbosa. Spores are discharged from the mature, black knots during spring rains. Optimal temperatures for spores to infect trees are 55 to 77 degrees F. This usually is the same time of bud swell. It can take two years for the gall to reach maturity and produce more spores.
Domestic flowering and fruiting cherry and plum species are common host plants, as are wild Prunus species. Some varieties are labeled with some resistance; none are immune. Check any woods and forest nearby for possible wild sources to be removed to protect plants in your yard.
Control measure are to cut out any knots or branch swellings at two to four inches below the infection site. Burn or put in the household garbage any pruned material to reduce amount of innoculum on site. Do not compost gall as they can continue to release spores which might cause reinfection. Fungicide can be used as a protection layer and should be applied at bud swell and stopped two weeks after full bloom. Follow label directions for number of times to use during the duration of susceptibility.