What's ailing my cherry tree?

Keeping an eye out for black knot

If you have a cherry or plum tree in your yard, you might have noticed an ugly black growth on some of its branches. This condition is known as black knot, a common fungal disease that can affect any tree in the Prunus genus (cherry, plum, apricot, peach, nectarine and almond), including both edible and ornamental varieties.

Here in Middle Tennessee, black knot is most commonly seen on plum trees, but also appears on some cherry trees. The disease is caused by a fungus, Apiosprina morbosa, which spreads when spores are released in spring.

The fungus grows within a branch and eventually encircles it with a hard, black gall that gives the condition its name. The leaves beyond the gall typically wither and die, which may be the first noticeable sign that something has afflicted your tree. Black knot can ultimately turn a beautiful, healthy tree into an ugly mess.

Black knot causes hard growths that resemble globs of tar on a tree branch.

Though a keen eye can spot black knot on a tree at any time of year, the best season to inspect your plum and cherry trees is winter, when the bare branches are more visible.

Unfortunately, there is only one effective treatment for black knot -- pruning to remove any branches with galls. This is best done in late winter, just before the tree begins putting out new growth. At least 3-4 inches below each knot should be pruned to reach healthy tissue. And all cuttings should be burned or disposed of to prevent spreading the fungus.

Black knot has caused significant branch dieback on this ornamental cherry tree in Green Hills. .

Keeping a close eye on all your trees can help to prevent small problems from turning into big ones. Subscribe to our newsletter to receive regular updates on tree care and tree policies in Nashville.