Black knot fungus is a disease that afflicts cherry and plum trees. It is similar to a spore comparable to a mushroom. Like most fungus this disease spreads in warm wet weather. In advanced stages this fungus can be detrimental because as it grows it wraps around branches, cutting off circulation. Affected branches eventually die, and in extreme and untreated cases it can kill a tree.
In the early stages you will notice swollen lumps on branches.
Spores begin to release during the leafing/budding period in early spring.
In advanced stages the swelling explodes into clumps.
The clumps may be brown or black and be velvety or brittle.
Click the read more link for pointers on Prevention
Because of the rapid spread of the disease, any tree in close vicinity should be treated.
Remove infected limbs before spring and before the tree is in leaf.
Remove each infected branch, make sure to prune each knot at least 2inch-4inch from infection site.
Knots that infect the main limbs of the trees should be assessed by an Arborist.