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.
Symptoms A rough, warty growth occurs above the soil line on dwarfing rootstocks. The growth generally is circular or spherical. Bumps are root initials (the beginnings of new roots). Knots can increase in size during the growing season. Small, normal-looking roots may be present if the burrknot is partially buried.
Trees with severe burrknot can be stunted or girdled or break off in high winds. Adjacent burrknots can increase in size faster than the normal trunk and can girdle the tree. A heavy crop load also can cause the tree to break at the knot. Burrknot has also been observed on the branches of susceptible cultivars in western Washington and Oregon.
Cultural control
- Avoid using rootstocks genetically disposed to this problem.
- Mound soil around the base of the tree to encourage the burrknots to root.
- Plant trees with the graft union 1 to 2 inches above the soil line. However, the risk of scion rooting increases with these soil-related control measures.
- Remove or kill the tissue with a knife or blowtorch. However, this generally is not an economical alternative.
Chemical control
- Gallex (ready to use) painted on very young knots to reduce further development. Knots may return the next year or, if treated late, may continue to develop. Tissue around the knot may be injured, especially on younger plants. Prepare the surface by removing soil from around the galled area, and allow the area to dry before application. Not registered in Idaho. 24-hr reentry.
Reference Rom, R.C., and Brown, S.A. 1979. Factors affecting burrknot formation on clonal Malus rootstocks. HortScience. 14:231-232.