Disease Affecting Plums and Cherries, Specifically Black Knot
Black knot disease is a serious problem for plum and cherry trees throughout the United States. This fungal disease primarily affects twigs, branches, and fruit spurs of plums, prunes, sweet, tart, and Mahaleb cherries, and occasionally apricots, peaches, and other Prunus species.
To prevent the spread of black knot, it's crucial to take certain precautions when planting new trees. Avoid planting them near or downwind from old or abandoned orchards with a significant black knot problem. Additionally, remove all wild plum and cherry trees from fence rows or woodlands next to the orchard site.
Effective control measures for black knot disease primarily involve pruning and sanitation. Infected branches and twigs bearing black knots should be pruned during dormancy (late fall or winter) before spring growth resumes, cutting about 10 cm below the knot to remove all infected tissue. Removing these nodules eliminates overwintering spores that would otherwise infect new growth. Incisions should be made with disinfected tools cleaned between cuts with 70% isopropyl alcohol to prevent spreading the fungus.
Preventative practices include regular annual monitoring for early signs such as slight bulges at the base of new shoots, and removal of nodules as they form during the growing season. Maintaining tree health with adequate watering, especially during drought, can help reduce susceptibility.
Regarding fungicide use, experts generally agree that fungicides do not cure established black knot infections but may help inhibit spore production. Fungicides are recommended only as an adjunct to good pruning and sanitation practices, particularly for protecting young or highly susceptible trees. They should be applied starting in early spring before bud break and continued at intervals until shoots mature.
Recommended fungicides include low-impact products with active ingredients such as sulfur, calcium sulfide, or calcium polysulfide (lime sulfur/gypsum). It's important to use fungicides labeled specifically for the Prunus species you are treating and suitable for edible cultivars if applicable. Organic fungicides may be used when green tissue appears and before and after flowering.
In addition to these measures, planting resistant or less susceptible Prunus species is recommended to reduce the risk of infection. Some plum varieties, such as Early Italian, Brodshaw, Fallenburg, Methley, and Milton, are less susceptible than Stanley. Japanese varieties of plums are generally less susceptible than most American varieties. Shiro, Santa Rose, and Formosa are much less susceptible, and President is apparently resistant.
The timing of fungicide sprays should be adjusted to account for inoculum levels and weather conditions. Pruning at least 2 to 4 inches below each knot is important because the fungus grows beyond the edge of the knot itself. New ascospores capable of spreading the disease may be formed in the young knots the year following infection but often are not formed until the second spring. Knots may become visible by the late summer of the year of infection but often are not noticed until the following spring, when they begin to enlarge rapidly.
If effective control measures are not taken, the black knot fungus can stunt or kill the tree. Ascospores are ejected into the air during rainy periods and are blown for moderate distances by wind currents.
In summary, the key control measures are regular pruning of infected branches during dormancy and growing season, tool disinfection between cuts, removal and destruction of pruned material, annual monitoring of trees for early signs, maintaining tree health with proper watering, fungicides as preventative sprays (sulfur-based or lime sulfur) starting early spring, not as curative treatments alone, and planting resistant or less susceptible Prunus species to reduce the risk of infection.
- In the realm of health-and-wellness for trees, effective pest management strategies for black knot disease revolve around science, such as pruning infected branches during dormancy and using fungicides like sulfur and calcium polysulfide.
- Engaging in fitness-and-exercise routines that ensure adequate watering can contribute to tree health, reducing their susceptibility to black knot disease and other medical-conditions.
- While exploring nutrition options for humans, consider the value of organic fungicides for gardeners looking to protect fruit trees from black knot disease, especially when green tissue appears and before and after flowering.
