Black Knot on Prunus sp. - Solution Guide
Black Knot
The black, knotty growth or galls that appear on plum and cherry branches are a fungal disease called black knot. Apiosporina morbosa is the fungal pathogen responsible for this disease.
Trees Affected
Black knot is common on Prunus spp., including ornamental and wild plums, cherries, peaches, and other stone fruits.
Black Knot Growth & Spread
Black knot fungus is spread by wind or splashing water. Once started, galls slowly expand and may eventually girdle and kill infected branches. Infected trees usually survive but are less attractive and produce far less fruit. Black knot is a cumulative disease that worsens with time, especially if left untreated.
- The hard-black galls result from an infection one or more years prior.
- Early first-year symptoms are small, light brown, or greenish swellings on branches as small as one-half inch long and one-quarter inch wide.
- As those swellings grow, they'll cover just one side of the branch. By the following spring, galls become dark olive-green with a velvety texture. By autumn of the second year, galls are larger, black, and extremely hard, typically growing entirely around the branch.
Note: Wood-boring insects sometimes invade older galls, and a different white or pink fungus may grow on them due to this secondary infection.
Control & Management
Once a tree has become infected with black knot, pruning out the galls is the best way to control the disease.
- Prune during the dormant season, either in late fall or winter, when trees are dormant. However, galls should be removed before April, when the fungus becomes active again.
- When removing black knot galls, make pruning cuts at least six inches below the galls because the fungus may have grown internally for some distance.
- Always disinfect pruning shears or cutting tools between every cut to prevent inadvertently spreading the disease from branch to branch or tree to tree.
- Copper fungicides or wettable sulfur will only be effective if used with the removal of existing galls.
- Fungicides should be applied in early spring, while trees are dormant, just before bud break until the bud color stage.
Note: Heavily infected trees that require heavy pruning may not be worth saving.
Note: Burn or bury all branches that get pruned off. Do not leave them on the property because the galls can still produce spores that can cause future infections.
Note: Fungicides are not curative; they are only preventative for further spread and infection.
Reduce the potential for black knot infections by reducing the number of wild Prunus spp. or cultivated trees that are already showing symptoms of black knot.
