Black Knot of Stone Fruits

By The Old House Web

Black Knot of Stone Fruits

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Black knot is common on ornamental plums, cherries and chokecherries and occasionally occurs on flowering peach and apricot.

Symptoms: The fungus forms large, hard, black, brittle knots of tissue on the outer surface of twigs. The knots nearly encircle the branch, forming a cylindrical mass that is 1/2 inch in diameter. The knots are actually abnormal growths of bark infected with the fungus. Infected twigs usually die within a year; larger branches die after several years.

Cause: Spores of the fungus form in the knots and are released and dispersed by rain and wind. The spores penetrate young shoots, usually in the branch crotches. Infections occur from bud break through leaf expansion when branches are wet with rain or dew. The knots appear in late fall and grow large the following early spring, turning black the second fall.

Control: Cut off infected branches. Burn all prunings of knotted parts before spring because spores from knots left on the ground can spread the infection.

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Black Knot on plum - 30K
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