Q: Many of the limbs and branches on my plum and cherry trees have 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. What do you think it is?
A: Your trees have black knot. It is common on plums and peaches in Georgia. The fungus causes a hard, black crust around twigs. Infection occurs from April through June, especially on the current season’s growth.
Remove all knots and swellings by pruning three to four inches below the knot during the dormant season before April 1. Plan a preventative spray of lime-sulfur (click for sources) or copper fungicide (click for sources) each year in winter.
See this factsheet.