Many
Prunus species, both edible and ornamental, are susceptible. Included are flowering almond, apricot, blackthorn, cherries (bird, bitter, black, mahaleb, Nanking, pin, sand, western sand, sour, and sweet), chokecherry, peach, and plum (American, beach, Canada, common, damson, Japanese, myrobalan, and Sierra).
The first symptom, which often goes unnoticed, is a swelling of the twigs of the current year's growth. Knots or galls develop from swellings on branches and trunks of infected trees. Bark on the galls splits, and galls appear corky and green. By the following spring, the galls are hard and black. They are obvious on branches in association with bent or curved twigs. Galls can be one foot or longer. Girdled branches usually die.
This fungus (
Apiosporina morbosa [syn.
Dibotryon morbosum]) overwinters as fruiting structures on the surface of knots. Spores may be released during wet weather from the time of bud break until terminal shoot growth stops, but mostly between white bud and fruit set stages. Infection follows on succulent shoots or wound sites. Swelling does not appear until the summer or fall after infection. Cankers continue perennial development on the stem, and two years after initial infection the knot has its first fruiting structures at the edges of the swellings. Spores are released and initiate new infections. In some cases, fruiting structures form in as little as one year after infection.
Wild plums and cherries are very prone to infection by the black knot fungus. Symptoms on wild Prunus may not always be obvious, so these species need to be carefully observed for knots. Infected wild trees should be removed. Remove galls on established trees during the dormant season, when galls are fully developed for the year. To be certain that all of the gall is removed, cut several inches below the knot. Sanitation measures are usually needed for adequate control of the disease, but may be supplemented with a fungicide program if the disease is unusually difficult to control. Fungicides can be used to help control the disease once sources of disease have been removed, but they will not be effective as the only treatment. Spray in the early spring before, or just as, the buds begin to swell. Where disease potential is high, consider additional applications of fungicides at white bud, petal fall, and again three weeks later. Once the disease is established, it may require two to three years of vigorous management to get it under control.
