Black knot, caused by the fungus Apiosporina morbosa, can infect cherries, chokecherries, and plums. Initially, infection occurs during wet periods in the spring when previously formed knots produce spores. These spores are wind blown or rain splashed over to the current season's growth or wounded tissue, where a new infection begins.
Fig. 1. Black knot galls. Upper branch is completely girdled by gall.
Photo: U of MN Plant Disease Clinic
Symptoms first appear in the fall as slight swellings or cracks in the outer bark. The fungus overwinters in the plant. The following spring, the bark ruptures and a light yellowish growth fills the crevices of the swollen area. By late spring, a velvety, olive green layer of spores covers the knots. During the second summer, the knots expand rapidly and by late summer or early fall, the knots are hard and black (Figure 1). Knots may increase in size yearly and eventually girdle branches.
Control of black knot is accomplished with pruning and possibly the use of a dormant season fungicide. In late winter before the new growth begins, remove all branches with swellings, cracks in the bark, or black knots. Prune at least three to four inches beyond the swollen or knotted areas, as the fungus may be further into the tissues than it appears. In the spring, continue to prune branches with new galls or swellings during dry weather. Thorough pruning and removal of infected branches, along with cultural practices that increase the vigor of the tree, may effectively control black knot without the use of fungicides. Wild plums and cherries may also be infected by black knot and can serve as a source of inoculum (spores) for adjacent trees. Where possible these knots should be removed.
In severe cases, apply fungicides during the dormant season before new growth begins, to help reduce inoculum (spores) available for new infections. Application of lime sulfur or Bordeaux mixture after pruning may also help reduce the number of new infections. Read the label carefully and apply only as directed.
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Chad Behrendt, Crystal Floyd