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 description sounds like the disease black knot. It is common on plums in Georgia. The fungus causes a hard, black crust around twigs. Infection occurs from April through June, especially on the current season’s growth.
See this Rhode Island factsheet.