Black knot disease is a fungal infection caused by Apiosporina morbosa. This disease primarily affects plums, with the potential to infect other prunus species such as cherry and peach. Black knot disease is characterized by distinctive black, warty galls that form on branches and twigs, which can eventually girdle and kill the affected limbs. Galls can range in size from a half-inch to several inches long. Trees with severe infections often become stunted and misshapen, resulting in significant yield reduction.

Symptoms and Causes

The most evident symptom of black knot disease is the presence of hard, black swellings or knots on the branches and twigs of the tree. Black knot disease spreads through spores released from mature knots during wet spring conditions. These spores are transferred by wind and rain where they can infect trees on new growth or wounded areas on the tree. The infection is more likely to occur during temperatures between 55 and 77 degrees F, with prolonged wet conditions facilitating spore germination. Infection can be observed during the late growing season as a small, brown swelling. The following growing season, the swelling will begin to develop a fungal growth, eventually hardening and turning a dark black color.

Prevention of Black Knot Disease

Regular inspection of trees is key to preventing the spread of black knot. While year-round inspection is important, the best time to identify these galls is during the winter when trees have dropped their leaves, making it easier to examine each branch. Prune out any galls, making sure to cut at least 6 to 8 inches below any visible infection. All infected plant material should be carried off site to reduce the inoculum in the field. Infected plant material can be burned or buried where allowed. Since black knot can also be found on native plums, removing any of these trees within 600 feet of desirable trees will help reduce the inoculum level and can help prevent infections.

Fungicides can be effective, especially when combined with proper pruning and sanitation practices. Fungicides should be applied preventatively using cover sprays, especially during the period of active shoot growth. Products containing chlorothalonil, captan, and thiophanate-methyl have shown activity against black knot. Adhere to all pre-harvest and re-entry intervals when selecting and applying chemicals. Managing black knot disease requires a strategic approach, utilizing both cultural and chemical strategies. Following pruning recommendations, thorough sanitation practices, and applying preventative fungicides are key to managing this disease in the southeast.