Disease

Black knot

Apiosporina morbosa

A fungus of plum, cherry and chokecherry that makes hard, black, knobby swellings on twigs and branches. Each knot grows larger every year and kills the branch beyond it, and old knots shed spores in spring rain.

Signs

• Hard, black, rough swellings around twigs and branches, from 1/2 inch to a foot or more long.
• New knots olive-green and velvety in early summer, hardening to black.
• Branches beyond large knots dying back.

What favors it

Wet springs, wild plum and chokecherry nearby, and susceptible plums such as Stanley.

Prevention

• Check plum and cherry trees every winter for knots, while the branches are bare.
• Remove knotted wild plum and chokecherry nearby, or prune their knots too.
• Plant less susceptible plums such as President, Methley or Shiro.

Crops it shows up on

Apple, pear & stone fruit; Shade & ornamental trees.

Often confused with

Sources

• Black knotUniversity of Minnesota Extension
• Black Knot of Plums and CherriesOhio State University Extension
• Black KnotMontana State University Extension

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