We usually see this disease on trees that form stone fruits, but black knot is also a common problem on ornamental Prunus species. Hosts include the ornamental plums and cherries that are often planted for their flower and foliage color. The disease usually rears its ugly head in the spring before the newsletter comes out, so we want to warn you about this disease now.
Black knot is caused by a fungus called Dibotryon morbosum, which infects new twigs in the spring. There is a slight swelling of the infection site by fall, but it will probably go unnoticed. The following spring (one year after infection), the swellings continue to grow and become roughened. Black knot is named for the elongated, rough, girdling, black swellings on twigs, branches, and sometimes the trunk. The knots become hard, brittle, and coal black. If the fungus growth is on one side of the stem, the stem may be bent at the knot. If the knot girdles the stem, the stem beyond will die. Because black knot galls are perennial, they continue to spread in the branch.
We donít believe that anyone intentionally buys diseased nursery stock, but inspect stems carefully for galls and swellings that may indicate early black knot infection. The older black knots represent at least two years of growth. Never buy trees with visible knots.
If you should find that your trees have this disease, take steps to get it under control using a combination of pruning and fungicide applications. Mark your calendar to prune in February. Remove all knots from the tree and burn, bury, or remove them from the site. Make cuts 4 to 8 inches behind any obvious black knot swellings. Actually, you can prune any time the tree is dormant, but if you wait until February you will be able to see all of the knots. Apply a dormant oil at bud swell.
Most infections occur between bud break and two weeks after bloom when wet conditions are accompanied by temperatures of 55 to 77 degrees F. For effective protection against this fungus, fungicide sprays should be applied as soon as buds open, and they must be continued every two weeks until about three weeks after petals fall. Many copper fungicides are registered for use against black knot, so pick a formulation that you prefer, carefully reading the label for host and disease clearance. Remember that early-season fungicide sprays prevent new infections but do not stop infections that are already present. That is why we recommend pruning. For more information concerning this disease, consult Report on Plant Disease No. 809, Black Knot of Plums and Cherries.