Learn how to identify and control black knot fungus, a common disease of plum, cherry, apricot and chokecherry trees.
for Black Knot
Black knot is a widespread fungal disease that attacks plum and cherry trees, both fruiting and ornamental. The fungus, Apiosporina morbosa, (also identified as Dibotryon morbosum and Plowrightia morbsum), singles out trees of the genus prunus, which includes peach, apricot, and chokecherry. Once established, black knot is easily identified with its hard, uneven, black galls that seem to enwrap twigs and branches. Black knot is a slow developer, taking a season before it’s visually apparent and producing spores. The trick to controlling the fungus is identifying the infection well-before the disease becomes firmly established. If left to grow, it effectively strangles new growth, girdling branches and dooming the tree to deterioration and poor fruit production. Insects and plant diseases use the galls as an entry to the tree.
The disease cycle starts when spores are released from established knots where the fungus overwinters. This occurs during damp spring conditions when temperatures reach 60 degrees or higher. The spores travel to other parts of the tree and, depending on the breezes, to nearby host trees. The spores germinate on stems beneath a thin film of moisture, often at the juncture of a new leaf start. They form small, olive-colored swellings over the first season, darkening in color as the season progresses, hidden by the leaves they’ll eventually kill,. By the second year, the galls are expanding quickly, especially where the weather remains humid. The growing infection begins releasing its own spores as it swells into the dark, easy-to-spot (especially after leaves have fallen) warty black fungus that coils along stems and branches. At this point, astute pruning and chemical treatments may not be enough to save the tree, no matter how careful the pruner is not to spread spores or leave them behind when removing the galls.
Paying close attention to your fruit trees and catching the infections as soon as they’re apparent, followed by quick pruning and careful disposal of the gall-infected branches, can save trees. Organic treatments can also help protect trees while keeping harmful chemicals off your fruit.
When choosing new plantings, consider that some varieties of tree and shrubs are more susceptible to the disease than others. They should be avoided in areas where the fungus is prolific. Tart cherry varieties are said to be less susceptible to the disease than sweet. Japanese plums are said to be less susceptible than American varieties. A number of plums, including President, Early Italian, Santa Rosa and Shiro carry varying degrees of resistance to the fungus. Susceptibility varies depending on the climate zones. Varieties that are susceptible in humid southern climates may be less so in dryer or cooler ones. Talk to your local nursery staff to see which varieties of plums, cherries, and ornamentals do best in your area. The Ohio State University Extension site has a chart on their black knot page that list the various levels for susceptibility to a number of plum and cherry tree varieties. (Not surprisingly, plum trees with resistance to black knot don’t do well in cold, northern climates.)
How to Control
- Inspect your trees carefully for first signs of the disease. This is best done in winter, when leaves are absent, but should be continued as well throughout the growing season. Look for cracks, discoloration, swelling, or other first signs of infection. Check carefully around twig and leaf axils.
- Remove any knots that are found. This is best done during winter when spore production is down. Cut well-past the galls, four to eight inches, to ensure all the infection and its spores are removed. Larger branches with established knots should be removed entirely. Use a pruning knife or chisel to remove galls on trunks and large branches, cutting down to the wood and out to at least an inch beyond the infection.
- Continue to inspect for and remove galls as the season progresses.
- Take care not to spread spores when pruning trees with black knot. Don’t allow twigs or other cuttings to fall to the ground where the spores could survive.
- Dispose of infected stems and branches by burying or, where allowed, burning. Small cuttings can be stuffed in trash bags and hauled away. Do not compost any infected cuttings unless your heap has an internal temperature of 160 degrees (not many do).
- Clean pruning tools as you use them with a solution of 1/2 cup bleach to a gallon of water. Wipe tools between cuts and leave your pruning blades in the solution for three to six minutes when finished. Or use a safe, commercial fungicide cleaner such as Physan 20.
- The Alberta Ministry of Agriculture and Forestry has a detailed guide to pruning and disposing of infected trimmings at their website.
- Fungicides can offer significant protection against black knot, but are unlikely to be effective if pruning and sanitation are ignored. Organic gardeners will want to avoid all but OMRI listed fungicides.
- Spraying trees with NEEM oil, a natural fungicide that controls leaf spot, rust, scab, and other tree fungus, can help inhibit the spread of black knot (it will not kill fungus that is already present). Spray trees per instructions just ahead of leaf and blossom emergence and, if possible, ahead of rain. Continue on a 7-10-day cycle until weather dries. Use of other fungicides can also discourage spores from germination. But few are specifically indicated for use on already infected trees.
- Spraying lime sulfur on trees during the dormant period is said to prevent the production of spores. Copper sprays applied during dormancy may also inhibit spore production.
- Take out wild cherry and plum trees around your property. They harbor the disease and release spores that are easily carried to your susceptible nursery trees.
- When planting new trees, place them away and upwind from established or wild prune and cherry trees.
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