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Black knot is visible as soft greenish knots or elongated swellings which form on the twigs and branches. The knots develop into black, corky, cylindrical galls that range from about 1/2 to 1 1/2 inches in diameter and may be more than 12 inches long. Branches beyond the galls are often stunted or dead.
Black knot is caused by a fungus that multiplies during wet spring weather. The visual symptoms are often seen 6 to 12 months after initial infection.
To control black knot, prune out an destroy infected twigs and branches in the fall and winter. Be sure to cut at least 4 inches below the visible signs of the knot. After each twig branch cut, be sure to sanitize your pruning tool in a mixture of 1 part bleach and 6 parts water as this helps to reduce the spread of the fungus. Large knots or cankers on main branches or the trunk should be cut out to a depth of at least 1/2 inch beyond the infection. In the spring spray the tree with either Captan Fungicide for Fruit & Ornamental Trees or the Fruit Tree, Vegetable & Ornamental Fungicide. Two or three more sprays at intervals of 7 to 10 days are required during the spring season.
Since an affected tree is under immense stress, it is also important to give the tree a good fertilizing. For general tree maintenance, proper fertilization and to improve the overall health of the tree, applying a TreeHelp Annual Care Kit is recommended. These kits contain appropriate fertilizer, mycorrhizal fungi and biostimulant, promoting root development and long-term vitality.