Garden Corner – Black Knot on Plum

Kelly Jackson
Christian County Extension Office

Black Knot on Plum

Black knot is a serious fungal pest of American, European, and Japanese varieties of plum and, to a lesser extent, sweet, tart, and Mahaleb cherries. This disease is very wide spread occurring on native wild plum and cherry trees which serve as a source of infection. Black knot becomes progressively worse each growing season resulting in stunted growth, death of branches and, eventually, the entire tree.

The black knot fungus causes elongated swellings or knots on twigs, branches, and fruit spurs. Symptoms first appear as small (about 1 inch diameter), olive-green swellings on the infected tissue during spring. In summer, they become darker and elongate. By fall, they become hard, brittle, rough, and black. The following growing season the knots enlarge, some growing to a foot or more in length. After the second year, the black knot fungus usually dies and is invaded by other fungi and various boring insects. Smaller twigs typically die within a year of being infected. Larger branches may live for several years before being girdled and killed by the fungus.

Disease is most likely to occur in the spring just after tree buds begin to open until the terminal growth ceases in early summer. Young growth (less than 3 years old) is most susceptible but older branches, especially those with injuries, can be infected. Black knot spores are released from overwintering knots on infected wood during rainy periods when temperatures are above 55 degrees. The spores are spread by wind to succulent green twigs where they enter the tissue and cause the plant to begin forming the turmor-like growths.

For the backyard orchard, the following tips are an integrated approach of controlling black knot problems.

  1. Purchase Disease-Free Stock:

Don’t bring home a problem. Inspect new plants for visible knots or swellings on the twigs and branches.

  1. Disease Resistance:

The more popular plum varieties, Stanley and Damson, are very susceptible to black knot. The varieties Early Italian, Brodshaw, Fallenburg, Methley, and Milton are reportedly somewhat less susceptible. Shiro, Santa Rose, and Formosa are much less susceptible. President is apparently resistant to black knot. Generally speaking, Japanese varieties of plum are less susceptible than most American varieties.

  1. Destroy Sources of Infection:

When planting a new plum or cherry tree avoid planting next to or downwind from wild plum and cherries in fencerows or woods as these trees are potential disease reservoirs. If possible, remove these trees. Scout your trees annually for the presence of black knot. Prune out infected twigs during the fall or winter before new growth starts. It is important to prune at least 2 to 4 inches below each knot to reach healthy wood. Because spores can develop and spread from knots left on the ground these should be collected burned or buried.

  1. Chemical Control:

Fungicide sprays can offer significant protection if applied timely and correctly. Chemical control of black knot should begin when the buds swell, temperatures reach 55 degrees or higher, and the foliage remains wet for six hours or longer. The critical spray period is from the time the flower buds show pink until two weeks after bloom. Although in areas where disease pressure is high, you may need to continue spraying until the terminals stop growing. During periods of frequent rainfall, repeat applications of fungicide every seven days. Available fungicides include basic copper sulfate, benomyl, captan, lime-sulfur, thiophante-methyl. Read and follow the label instructions for rates and precautions. In evaluating control programs, remember that knots often do not become apparent until the year following infection.