Problem type: Disease
Name of problem: Black Knot on Prunus species
Plant name(s): Plum and cherry trees; Native and Shubert chokecherry
Symptoms / Characteristics:
The characteristic feature of black knot is the presence of thick, black, irregular swellings on twigs and branches. The galls are often noticed in the winter when leaves are not present. The disease is difficult to notice during the early stages of infection. Initially, the disease appears as small light brown swellings on current or previous seasons growth. The following year, the swellings appear olive green with a velvety texture. By the end of the season, knots darken and harden. Numerous knots may be present on one tree. Often the branch beyond the knot will either fail to leaf out or wilts suddenly.
Black knot is caused by the fungus Apiosporina morbosa, which infects trees in the spring, about the time of bud emergence. Spores are released following a period of warm, wet weather. Only a few hours of rainfall are needed for dispersal. Temperatures between 16 and 27°C are ideal for dispersal, germination and infection. Spores are spread by splashing water, wind, insects and birds.
The noticeable black gall-like growths do not become evident until the next year in late summer to fall. The following spring after infection, the outer bark splits, revealing a yellow to olive green fungal growth. In late spring, as the knots begin to develop, growths are covered with olive green velvety tissue made up of spores and spore forming structures. Often, a diseased branch will bend sharply at the knot due to one-sided growth. Knots then develop very slowly and may appear as only small galls by the end of summer. The gall has a corky texture and becomes hardened and black. The black knot fungus overwinters in the knots, but gall enlargement ceases over the winter and resumes again in the spring when knots may then enlarge rapidly. Old knots enlarge every year and may range from ½" to 1 ft. in length. Duration of the disease cycle is usually 2 years. Fungus in old knots may invade other tissues to form new knots. The fungus can also spread internally. Branches will likely by girdled stopping the transport of water and nutrients, and dieback may occur. A branch may survive, but may have a large canker with a sunken center serving as an invasion point for other insects and diseases. Succulent new growth or wounded tissue is more commonly infected.
Control / Preventions:
Prune infected branches at least 4-6" beyond the gall. Prune trees when they are dormant, before March 1 or after they are done flowering. Sterilize tools between each cut using chlorine, bleach or alcohol. Knots are capable of producing spores after removal so burning, burying or removing infected branches from the site is necessary. Maintain healthy trees and avoid stress on the tree by watering and fertilizing when needed.
In fall when trees are dormant or in spring before bud break, apply a lime-sulphur spray according to the label. In spring, after pruning, spray tree with Captan, or a fungicide containing benomyl just before buds open. Repeat the application twice more at 7-10 day intervals. An application can be made at full bloom if the chemical used does not contain insecticides. Early spring spraying is important to protect new growth. During wet weather, apply fungicide every 3-5 days.
Chemicals are ineffective if cultural practices are not carried out. Black knot may require two to three years of management to bring it under control. Complete eradication is currently not possible.