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Black clumps on tree branches

Black Knot on my treeYour plum, cherry or choke cherry tree may have a disease known as black knot. The characteristic feature of black knot is the presence of thick, black, irregular swellings on twigs and branches. 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 season’s 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 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 2-30cm 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.

What you can do

In order to control black knot, prune infected branches at least 15-20cm beyond the gall. Prune trees when they are dormant, before March 1 or after they have finished 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.

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