Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001699
11/12/99
Black Knot of Stone Fruits
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 (Vis. 1) 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. 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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This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
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