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Significance
Black knot is a disease that is most severe on cultivated crops in orchard and ornamental planting situations. Black knot appears to be a minor problem on Prunus species found in forest situations, where susceptible trees are surrounded by many nonsusceptible species of trees.
Black knot disease is mainly a problem in North America (Canada, the United States and Mexico) where it is indigenous. A record from 1979 indicated the presence of the fungus on pear in Taiwan but no other incidences have been reported from Asia. Currently it is not found in Europe or the EPPO region. The EPPO region consists of 43 European and Mediterranean countries that are responsible for international plant protection in their region. Apiosporina morbosa is listed as an EPPO A1 quarantine pest. Inclusion on the list requires the countries to follow phytosanitary regulations and to make appropriate requirements for admission of plant material in their countries.
Black knot has been reported on 24 species of Prunus but is most commonly found on wild and cultivated plums and cherries. Early publications from the 1950's describing the disease report no cases affecting peach, but a few rare infections have been reported since that time. The disease can be found throughout North America but is most commonly found in the northeast. It was first reported as a destructive disease in Massachusetts in 1811. It was first described in 1821 by L. D. Schweinitz from specimens collected in Pennsylvania. Researchers believe the disease is caused by a native pathogen that was only found in the northeastern states until around 1875, when observations of the disease began arising in the central states.
Young, infected twigs may die during the first year of infection. Larger branches may take several years to display severe damage. The infected trees decline and become more symptomatic with each growing season. The infection stresses the entire tree making it weaken, decline, and possibly die. The stress placed on the tree may also make it susceptible to infections by other pathogens.
Economically the trees lose value after a few years. The portions of a branch distal to a knot become stunted, and occasionally knots enlarge to girdle a branch and possibly kill it. Trees with multiple infections become dwarfed and misshapen, markedly reducing their productivity and attractiveness (figure 18).
 figure 18 |
Because of the long infection process and disease cycle, this disease is often overlooked by home gardeners and fruit producers. The leaves can mask the symptoms until firmly established infections are in place. Once established, it is very difficult to manage the disease. Commercial growers often discover the disease more quickly because they regularly inspect their trees during routine crop management. Awareness and strict monitoring of susceptible plants should be a priority for all home gardeners and commercial growers.
Copyright © 2002
by The American Phytopathological Society