Description: Black knot is a serious disease of plum and cherry trees caused by the fungus Apiosporina morbosa. It is referred to as black knot because that is the main symptom of the disease.
Symptoms: Black knot infections are caused by spores that attack the current year’s growth, mainly twigs and branches, although it can also enter wounds. When the spores land on the new growth it takes several months before a light brown swelling appears, which ruptures as it enlarges. By the next spring the warty swelling turns olive green. The young knots turn darker and elongate by summer. By fall they are hard, brittle, rough and black. They eventually expand enough to girdle and kill affected twigs, branches and entire trees.
After the second year the original fungus dies and secondary fungi move in. The black knots can turn white or pink during the summer. Boring insects also move into the knots, further weakening the tree.
Management of black knot begins with selecting resistant cultivars. In our area, purple leaf plum and wild cherry are highly susceptible to black knot. Spores can be spread by wind or rain. Scout natural areas nearby and remove any sources that would affect your tree. Visually inspect your trees every year for symptoms of black knot. Prune out and destroy any evidence of black knot before new twigs begin to grow. It is important to prune at least four inches below the knot itself to make sure all of the fungus has been removed.
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