Black Knot on Cherries & Plums

by Dr. James H. Blake, Director, Home & Garden Information Center, Clemson University, 2009

Hi, I'm James Blake, Director of the Clemson Extension Home & Garden Information Center. Today we're looking at a disease known as black knot.

We’re along the edge of the Clemson University Experimental Forest looking at a wild black cherry with some strange black growths on the twigs. This disease is known as black knot caused by the fungus Apiosporina morbosa. It infects cherries and plums.

The fungus can infect green shoots but can also enter through wounds. Infection leads to these long, rough galls on the twigs and branches and can even infect the trunks of trees. These galls can range in size from an inch to more than 20 inches. These galls can form on one side of a branch or can encircle the entire branch. When infection circles the entire branch or twig it leads to death beyond the point of the gall.

The duration of the complete cycle of this disease is normally 2 years. After infection occurs in the spring, swellings appear in the fall on the current season’s growth. After being dormant during the winter, the fungus resumes growth in spring and the bark splits revealing olive-green fungal tissue covered with spores. These galls turn darker during the summer and fall and then turn hard and black during the winter. In the following spring, another spore stage is produced on the black knots that starts new infections.

Management of black knot begins with the selection of resistant cultivars. If any galls form on susceptible varieties these galls should be removed 4 inches below the point of the gall. All infected plant material should be destroyed.

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