Black knot is a common, often serious, disease of plums and cherries in Kentucky. Trees in both commercial and residential plantings are susceptible to the disease. Black knot results in knotty growths that, over time, encircle limbs and result in branch death. Pruning and sanitation are the primary means for reducing or eliminating black knot.
Black Knot Facts
- Symptoms begin as small, light brown, irregular swelling or knots on limbs (Figure 1). The next year, enlarging knots become olive-green with a velvety surface and progress to become hardened, brittle, black swellings (Figure 2). They can reach lengths of 6 inches. Knots continue to expand each year until girdled branches eventually die.
- Only actively growing twigs of the current season’s growth are susceptible.
- While infection takes place in spring, knot development is not evident until autumn.
- The pathogen overwinters in knots on previously infected twigs and branches, and spores are spread by wind and rain.
- Caused by the fungus Apiosporina morbosa (syn. Dibotryon morbosum).
Cultural practices such as pruning and sanitation are the primary means for reducing or eliminating black knot.
- Prune out knots in autumn or winter after leaves fall and when infected branches are easy to recognize. Inspect trees again in April and remove any newly formed knots.
- Remove any trees with girdled trunks and/or large limbs.
- Destroy or discard all diseased wood.
- Remove wild plum and cherry trees in the vicinity.
Fungicide applications may be used in conjunction with the cultural practices listed previously, but are often not warranted.
- Fungicides can be applied in spring to protect young, expanding twigs.
- Homeowners should refer to Extension publication, Disease and Insect Control for Home Grown Fruit in Kentucky (ID-21) for specific fungicide recommendations.
- Commercials growers should refer to the Midwest Fruit Pest Management Guide (ID-232) for current fungicide recommendations.
- Always follow all label directions when utilizing fungicides.
- Black Knot (PPFS-FR-T-04)
- Backyard Peach & Stone Fruit Disease, Pest, and Cultural Practices Calendar (PPFS-FR-T-22)
- Commercial Fruit Pest Management Guide (ID-232)
- Disease and Insect Control Program for Homegrown Fruit in Kentucky, including Organic Alternatives (ID-21)
- Homeowner’s Guide to Fungicides (PPFS-GEN-07)
By Kimberly Leonberger, Plant Pathology Extension Associate, and Nicole Gauthier, Plant Pathology Extension Specialist