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Black knot

Apiosporina morbosa (Schwein.:Fr)

Black knot usually develops over two seasons. The disease first appears in late summer or autumn as an olive-green swelling on new shoots (A). Disease develops rapidly the following summer, forming a characteristic dark, course-textured warty knot (B, C). Knots vary from 2.5 cm to nearly 30 cm in length and may or may not encircle the branch. The vascular tissue becomes restricted in infected branches, ultimately leading to the death of the branch.

Distribution

Widespread; the disease is very damaging on many varieties of plum, less so on sour cherry.

Similar Species

Black knot is often unmistakable.

Management

Black knots should be pruned from infected trees and removed from the orchard during the dormant season. Wild plum, prune, and cherry seedlings should be removed from fence rows, woodlots, and the orchard perimeter, as they serve as a source of the disease. Fungicides should be applied to protect trees between white bud and petal fall growth stages, but treatment is likely to be ineffective if pruning and sanitation are not practiced.

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