Issue 4, May 21, 2018

Black Knot

Black knot is a common fungal disease that affects at least 25 Prunus species, both edible and ornamental. The disease is caused by the fungus Apiosporina morbosa (also known as Dibotryon morbosum), which infects the new twigs, branches, and fruit spurs during the spring. Trunks also can become diseased. Most infections occur between bud break and 2 weeks after bloom when wet conditions are accompanied by temperatures of 55° to 77°F.

Early symptoms of the disease are easily overlooked and appear in the autumn as swellings of the current year's growth. Swelling of infected branches is arrested during winter dormancy, and resume the following spring. The bark on swelling branch eventually ruptures revealing corky, olive-green fungal tissue. By the fall, affected tissues are hard, brittle, rough, and darken to a characteristic coal black color. Affected branches often fail to leaf out the following spring or wilt and die by early summer. The infected branches that remain living have black knots that elongate on a perennial basis. Some knots can develop to be one foot or longer. The disease becomes more severe with each growing season. Black knot does not typically kill a tree but causes deformed growth if left unchecked.

Branch girdled by a black knot infection. Photo courtesy of James Schuster.

Disease Management Options:

  • Purchase disease free plants. Carefully inspect the plants prior to purchase. Avoid plants with visible knots or abnormal swelling on branches and twigs.
  • During the dormant season (late winter or early spring), prune out infected branches. Make cuts 4 to 8 inches below any obvious infected tissue. Destroy (burn) or bury affected branches
  • Remove and destroy any unwanted Prunus species that may be harboring the pathogen. Wild plums and cherries are more susceptible to black knot than cultivated varieties. If your landscaped area is near a wooded site, look for galls on the wild Prunus species. Infected wild trees should be removed.
  • Fungicide sprays should be applied as soon as buds open and must be continued every 2 weeks until about 3 weeks after petals fall. Many copper fungicides are registered for use against black knot. Carefully read the product label to ensure that it has been approved for the host and disease. (Travis Cleveland)

Travis Cleveland

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