A fungus causes Black Knot disease in both plum and cherry. The spores are produced from early spring through early summer and are spread by wind and splashing rain. When spores contact young wood, they germinate and penetrate the bark either directly or through wounds. Infection generally occurs during warm wet weather, from the end of April through June. Remember, all the rainy weather last spring and summer, it was the perfect condition for the development of the disease.
First symptoms are small, light brown swellings of the current or last season's growth.
These swellings may be difficult to see and often go unnoticed. The disease becomes evident only after the knots enlarge and become olive-green with a velvety texture. Soon after this stage the knots become darker and by fall they appear as black hard knots. These hard, coal-black knots are especially noticeable this time of year when all foliage is gone and the trees are bare. The knots will continue to grow until they girdle the branch and kill it.