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Black Knot of Plums, Prunes & Cherries

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Black Knot

Question of the Week

Black Knot of Plums, Prunes & Cherries

Black knot infections are very obvious against a backdrop of winter snow. Q. I have a Canadian Cherry tree that's 10-15 ft. tall and about 12-13 years old. Over the last three to four years it started developing what looks like a lumpy black growth on its limbs that's getting worse every year. The last couple of years we pruned the branches that had this growth, but now it is all over the tree! Is there any treatment available for controlling it?

A. You've provided a perfect description for a common disease of more than twenty different cherry, plum and prune trees called "Black Knot."

If you look very closely at a black knot in ealry spring, you can see the structures that contain ascospores are swollen and ready to burst as the weather warms.The part of the fungus that infects new growth each spring are called ascospores. Millions of ascospores, which are black, cover the surface of each growth, left. In the spring rainfall splashes the structures from the growths, or "knots," onto newly emerging stems.

New growth in the upper left corner of this photo has likely been infected by ascospores from the obvious infection in the center of the picture.Infection of new, green shoots occurs from April through June during periods of warm, wet weather, at right. The fungus grows through the soft bark and spreads among cells within the shoot. It also releases a growth promoting chemical, causing the shoot to begin to swell and often bend in unusual directions.

The characteristic black, knot-like growths actually become visible during late summer and autumn a year and a half after the initial infection. Because ascospores from one and one-half year-old knots infect new growth each spring, itís important to prune all visible knots out of infected trees when it's above freezing in January, February and March. Because infected tissue can exist a number of inches below visible knots, pruning cuts must be made at least eight inches below the black, visible portion of each infection.

Also keep in mind that infectious ascospores can be released from infected twigs and stems even after they've been removed. So, it's important to burn, bury or otherwise dispose of all prunings.

Where large numbers of wild cherry, plum or prune grow nearby, the application of fungicides has been recommended in the past to protect succulent new growth until the period of infection period ends in mid-to late June.

However, at this time (April of 2007), there are no fungicides labeled/recommended for use in New York for controlling black knot infections on bearing fruit trees, or in landscape plantings.

Therefore, where infection sources (wild cherries, plums and prunes) are common the only practical option may be to remove heavily infected trees and avoid replanting with susceptible species.