Black knot is a fungal disease that attacks cultivated and wild plums, cherries, and other Prunus species. Despite the wide host range, there are host-specific strains that will infect certain species and not others.
BIOLOGY: Black knot fungus forms elongated swellings or knots on twigs and branches. In the spring the fungus ejects spores that are splashed or blown onto the current season’s growth. Although infections can occur anytime after bud break when shoots are actively growing, most infections occur from just before bloom until after petals fall. Optimum conditions for infection are wet weather between 60°F and 80°F.
After a spore germinates, it penetrates the tree’s tissues and stimulates the cambium to grow abnormally, producing a tumor-like knot that expands rapidly in length and girth. Eventually the swollen bark tissue splits open to reveal a soft, corky, olive-green fungal mass. This darkens over the course of the season and by autumn turns into a hard, woody, coal-black growth.
The knots become mature two years after the time of initial infection. They are then capable of producing spores. The active margins of the knot will continue growing and enlarging.
SYMPTOMS: Hard, cylindrical or spindle-shaped black galls occur on twigs, branches, and fruit spurs. Occasionally they occur on the trunk. These can be half an inch to two inches in diameter and over a foot long. Heavy infestations lead to twigs and branches dying after being girdled by the fungus.
CONTROL: Before bud break, in late winter, prune away knot-infested twigs and limbs. Try to make the cut six inches below the knot. When knots are present on major scaffold limbs or on the trunk, remove them by cutting out the diseased tissue down to bare wood and at least four inches beyond the edge of the knot. Burn or bury the infected wood, because the knots are capable of sporulating for some time after they are removed. Fungicides will not provide complete control without pruning. Lime-sulfur can be used as a dormant spray. Chlorothalonil (Daconil 2787) can be applied until the shucks begin to fall from the fruit. Applications of Captan or thiophanate-methyl (Cleary’s 3336) can be made just before the flowers open and as cover sprays during the season. Check the labels for precautions and application times to avoid phytotoxicity.
Black knot-resistant plums include the European plum Prunus domestica ‘President’ and the Japanese plums P. salicina AU-Rubrum‘, AU-Rosa’, and AU-Cherry’. Moderately resistant Japanese plums include P. s. ‘Formosa’, ‘Santa Rosa’, and ‘Shiro’.