Black knot fungus Apiosporina morbosa
IDENTIFICATION – When the leaves are off the trees, these grotesque, black swellings are easy to spot. You’ll find them mainly on members of the Prunus family — cherries, plums and peaches. Fungus spores spread by wind, insects, birds or humans. Black knot grows in warm (55 to 75 degrees), humid spring weather. The swellings develop over a two-year period. Even though infection begins the first year, no real symptoms appear until the following spring. Then the bark on a stem splits and a green fungus appears. It could cover an area 12 inches long and be several times thicker than the branch it infects. This green fungus hardens and turns black and knotty.
DAMAGE – These growths, or knots, shut off water and nutrients to the branch, which eventually wilts, dries up and dies.
CONTROL – The best defense against black knot is to plant resistant varieties. Check with your extension service or nursery for recommendations. During the fall and winter, prune and destroy infected branches. Cut at least 4 inches below the swollen, black area. To prevent spreading, dip pruning tools in a 10-percent bleach solution between cuts.