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Black Knot Tree Disease
Home  > Tree Care  > Tree Diseases  > Black Knot Tree Disease

BLACK KNOT

Black Knot is a common, and destructive, disease of fruit and ornamental trees in the Prunus species such as Plum and Prune trees. Black Knot causes black lumpy growths to develop on twigs and branches. Once a tree is infected with Black Knot it will become progressively worse if no action is taken. Branches will gradually lose vigor, flowering will be reduced, and eventually the tree will be killed.





TREES INFECTED BY BLACK KNOT

Black Knot can infect American, European, and Japanese Plum and Prune trees, wild cherry trees, and on ornamental Purpleleaf Plum trees. It is occasionally found on Apricot trees, Peach trees, Sweet Cherry trees, and Tart Cherry trees.

If you want to plant a Plum tree for fruit production be forewarned that the following varieties are susceptible to Black Knot:
  • ‘Bluefre’ Plum
  • ‘Bradshaw’ Plum
  • ‘Damson’ Plum
  • ‘Early Italian’ Plum
  • ‘Fellenburg’ Plum
  • ‘Methley’ Plum
  • ‘Milton’ Plum
  • ‘Shropshire’ Plum
  • ‘Stanley’ Plum

Plum varieties resistant to Black Knot are:
  • ‘Formosa’ Plum
  • ‘President’ Plum
  • ‘Santa Rosa’ Plum
  • ‘Shiro’ Plum

Black Knot fungus. Photo: Mike Schomaker, Colorado State Forest Service, Bugwood.org

A tree with a severe case of Black Knot fungus. Photo: Joseph O'Brien, USDA Forest Service, Bugwood.org

Black Knot fungus. Photo: Robert L. Anderson, USDA Forest Service, Bugwood.org

Black Knot fungus. Photo: Joseph O'Brien, USDA Forest Service, Bugwood.org

SYMPTOMS OF BLACK KNOT FUNGUS

Black Knot displays itself as elongated, lumpy black swelling on twigs and small branches. Soon after infection the knots are olive-green and soft, but turn black with age. Knots can vary from one inch to one foot in length. On heavily infected trees the knots can be found on larger branches and even on the tree trunk.

Eventually, the knots will encircle and kill the end of the branch they are growing on. The knots are also an entry point into the tree for insects such as borers.

DISEASE CYCLE

Cool, wet weather favors infection by Black Knot. The highest level of infection with Black Knot occurs when temperatures range between 55 and 75 degrees Fahrenheit, but infection can take place at lower temperatures. New spores are produced on the knots and are dispersed in the wind to newly emerging shoots. New knots can occasionally be found in the late summer or autumn following a spring infection.

HOW TO CONTROL BLACK KNOT

The best control of Black Knot is achieved with a combination of tree pruning and tree spraying. It may require several years of tree pruning and spraying to eradicate Black Knot and re-infection from surrounding trees is always a possibility.

Cultural Control of Black Knot

Pruning of Black Knot infected branches can be done from late fall to early spring. If possible, tree pruning should be done before the spring growth starts and temperatures reach 50 degrees Fahrenheit. All branches with knots should be removed, cutting back to lateral branches. Pruning cuts should be made 4 - 5 inches below the knot because the fungus grows in the wood beyond the edge of the knot. Pruning tools should be sterilized with bleach or Lysol between each pruning cut to prevent spreading the fungus. Infected branches should be burned, buried, or disposed of in the garbage.

If possible, remove nearby wild Plum, Prune, or Cherry trees that could serve as a potential source of Black Knot spores. At a minimum the knots from nearby wild trees should be removed.

Fungicide Control of Black Knot

Several fungicides are labeled for control of Black Knot. A fungicide program is not likely to be effective unless old knots are removed. Whenever possible, apply fungicide treatments before a rainy period and when temperatures are above 55 degrees Fahrenheit.

Read fungicide labels carefully as some fungicides may cause leaf injury to certain Plum tree varieties.

 

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