Plum, Peach, Nectarine – Black Knot Disease

Q: We have a fruit cocktail tree, bearing plum, peach and nectarine fruit, planted five years ago. It blossomed and had some fruit but the fruit developed a gum-looking disease and eventually fell off. A good number of branches on the tree are infected with black, hard, scaley cases encircling them. I’ve tried to cut into them but it takes a strong knife. It’s almost like they’re part of the tree. 

A: I don’t have good news. All of the fruit on your grafted tree are susceptible to a disease called black knot and the affected limbs must be pruned out.

Infection occurs from April through June, especially on the current season’s growth. It is common on plums and peaches in Georgia.

Black knot is a disease that gets progressively worse each year unless controlled, and it will eventually stunt or kill the tree. It is frequently seen in the woods on wild black cherry (Prunus serotina)

Remove all knots and swellings by pruning three to four inches below the knot during the dormant season before April 1. If the tree is small, the pruning may remove most  of the tree. Remove  the infected tree and burn or put in your yard waste for removal.

No fungicides are effective for control. Chlorothalonil (Daconil) will protect uninfected parts of the tree Read labels to check.

See this  factsheet