Tree Diseases - Black Knot
Black knot is a common and often serious disease of Plum and Cherry trees in New England. Once established, the disease becomes progressively more severe each year unless control measures are taken. Infected limbs and twigs lose vigor and may eventually die, and commercial plantings in which the disease becomes widespread are seldom economically viable to maintain. Black Knot can take over the foliar crown of a tree very quickly with the right weather conditions. Black knot is occasionally found on peaches, and apricots but is seldom a problem on these trees.
The fungus over winter's in knots on twigs and branches or in the infected wood immediately surrounding them. In the spring, infective spores (ascospores) are produced in sacs contained within tiny fruiting bodies on the surface of the knots. These ascospores are ejected into the air during rainy periods and are blown for moderate distances by wind currents. Only succulent green twigs of the current season's growth are susceptible to infection. Ascospores that land on them may germinate and cause infection if the twigs remain wet for a sufficient length of time.
The value of these preliminary steps for increasing a trees longevity cannot be overemphasized. Established trees should be scouted each year for the presence of black knot, and infected twigs should be pruned and burned or buried before bud break. It is important to prune at least 2-4 inches (5-10 cm) below each knot because the fungus grows beyond the edge of the knot itself.
Fungicide programs can offer significant protection against black knot but are unlikely to be effective if pruning and sanitation are ignored. Fungicide application will only work on a tree that has very few Knots. Often by the time we get there the trees are completely covered and control is not an option.
Normal growth is disrupted in the infected regions, and a knot is formed as the fungus causes the plant to produce tumor like growths. Knots may become visible by the late summer of the year of infection but often are not noticed until the following spring, when they begin to enlarge rapidly. New ascospores capable of spreading the disease may be formed in the young knots the year following infection but often are not formed until the second spring. The fungus continues to grow in infected wood during the spring and fall months, causing the knots to elongate several inches each year and eventually girdle affected twigs and branches.
The primary goal of any black knot control program should be to limit the amount of ascospore inoculum available to cause infection. When establishing a new Plum or Cherry planting, avoid planting trees next to or downwind from an old or abandoned orchard with a significant black knot problem. Similarly, remove all wild Plum and Cherry trees (potential disease reservoirs) from fencerows or woodlands next to the planting site.
Ascospores are potentially available from the time of bud break until terminal shoot growth stops. Although the precise environmental conditions required for infection are uncertain, only a few hours of rain apparently are required at temperatures above 55° F.