Cause Genetic. The knot is a highly branched but dwarfed root system above the soil line. Although it is often mistaken for crown gall, it is composed of differentiated root tissue that starts from root initials in the young tree. Low light and high humidity and temperature can work together to stimulate development of the root initial. Trunk protectors that reduce light can increase the occurrence of burrknot. Burrknot is common on many of the popular rootstocks such as M7, M9, M26, MM106, and MM111.
