Black Knot

Need to know 

• This disease is caused by the fungus Apiosporina morbosa and typically affects both wild and cultivated Prunus tree species.
• Black knot causes galls (or swellings) to occur on branches, twigs, and/or the main trunk.
• Fungal spores produced on year-old or older galls initiate infections on young, succulent twigs or wounded tissue during wet conditions in the spring.
• The disease is controlled primarily through good sanitation measures.

Overview of black knot 

This disease is caused by the fungus Apiosporina morbosa and typically affects both wild and cultivated cherry, chokecherry and plum species. Black cherry and pin cherry are not susceptible to black knot.

Symptoms of black knot 

Black knot causes galls (or swellings) to occur on branches, twigs, and/or the main trunk. Galls typically appear soft and olive green when newly formed, but eventually become hard and black with age. Several galls can infect a single tree and usually causes branch death beyond the area of the infection. These galls, which are longer than wide, may extend up the branch from a few inches to a foot or more. They are especially noticeable before the leaves emerge in the spring.

Signs of black knot 

The microscopic fungal bodies of the pathogen Apiosporina morbosa reside inside the galls and can only be seen using a microscope on thin gall sections.

Disease cycle of black knot

Black knot is caused by the fungus Apiosporina morbosa. Fungal spores produced on year-old or older galls initiate infections on young, succulent twigs or wounded tissue during wet conditions in the spring. Often the knots become covered with a whitish secondary fungus or insects.

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us.

Want to submit a sample? Follow the instructions at Submitting Trees and Shrubs. 

Management of black knot

The disease is controlled primarily through good sanitation measures. All galls should be pruned out and destroyed, preferably when the tree is dormant. It is important to make cuts at least 2-3 inches below the swelling because the fungus may extend beyond the swelling.

Sanitation measures usually control the disease adequately, but fungicides such as lime sulfur or tribasic copper sulfate may aid in the control of black knot. Several applications need to be made beginning before bloom and continuing until after fruit set. Fungicide use alone will NOT control the disease. Always read and follow all label directions when using pesticides.

It is also helpful to remove any diseased wild plum or cherries in the vicinity, such as in fence rows or nearby wooded areas.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.