Black Knot of Cherry
From Bugwoodwiki
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USDA Forest Service. 1979. A guide to common insects and diseases of forest trees in the northeastern United States. Northeast. Area State Priv. For., For. Insect and Disease Management., Broomall, PA. p. 123, illus.
Black knot of cherry is caused by the fungus Apiosporina morbosa, formerly called Dibotryon morbosum. Black charcoal-like tumors form on the hosts, which include all varieties of cherry, plum, and peach. Young shoots can be killed in a year, but the larger branches generally resist the attack for several years. On the trunks of larger cherry trees, burls or swellings up to 6 feet long can form. Most of the trunk can become large enough to render the wood unusable.
Infection occurs in spring, but the swelling that follows only becomes evident the following year. Soon after becoming visible, the swelling takes on a pale-green color, then cracks and becomes covered with an olive-green down. This downy coat is made up of asexual spores of the pathogen, which are dispersed by the wind. At the end of summer, these spores stop forming and the olive-green down disappears. The swelling hardens and progressively blackens. In winter the ascospores (spores of the sexual stage) develop in small, globular fruiting bodies that form on the knots, giving them a gnarled appearance. The following spring these ascospores are dispersed by the wind to neighboring healthy branches. New infections are produced, and the 2-year cycle is thus completed.
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 Photo by Randy Cyr, Greentree, Bugwood.org |
