How to Manage Pests
UC Pest Management Guidelines
Crown gall appears as rough, abnormal galls at or below the soil surface on roots or trunk. Live galls are not hard but soft and spongy. The centers of older galls decay. Young trees become stunted. Older trees often develop secondary wood rots.
Crown gall is caused by a bacteria, Agrobacterium tumefaciens, that survives in soil and gall tissue. Bacteria enter primarily through wounds. Crown gall is most damaging to trees that are 1- to 8-years old. Paradox rootstock is especially susceptible.
The incidence of crown gall can be reduced by planting noninfected, "clean" trees. Before planting, make sure trees stay moist and the roots do not dry out. It is also important to carefully handle trees to avoid injury as much as possible, both at planting and during the life of the tree in the orchard. Although preplant, preventive dips or sprays with a biological control agent are available, their effectiveness can be variable on walnut trees.
Look for and treat crown gall during the growing season when the orchard is not wet because moisture favors the bacterium. When established orchard trees are infected with crown gall, you can use a combination of surgery, flaming, or a bactericide to treat the tumors. The best time to treat is in the spring because with rapid tree growth occurring, new callous tissue is formed relatively quickly.
The treatment is most effective for small galls on young trees. The procedure, however, can be expensive and difficult to carry out, depending on the size and location of the galls. If trees less than 4 years old are severely affected with galls, it is more economical to remove the trees and replant. If galls develop on trees after they are 7 or 8 years old and the trees appear healthy otherwise, then treatment probably is not necessary. For trees between 4 and 7 years old, the decision whether to treat galls or remove trees depends on the severity of galling and the cost of treatment relative to the cost of replacing trees.
To treat crown galls, first remove soil away from the crown and roots to completely expose the gall. Soil can be safely removed using pneumatic equipment such as air compressors or hydraulic excavation with fire suppression equipment or water washers. To flame the gall, use a propane cylinder or bottle and slowly move the torch tip around the margin of the gall, creating a red-hot zone that is about 1 inch wide. It may be advantageous to surgically remove the gall in order to gain access to all parts of the gall margin. If surgery is used, be sure to sterilize the tools with heat before advancing to the next tree. Leave the treated areas uncovered for a year and re-treat if galls begin to regrow. Treatment success is about 80%.
As an alternative to flaming, galls can be treated with a bactericide such as Gallex, but treatment success is dependent upon complete removal of the gall first.
When replanting a previously affected site, remove as many of the old tree roots as possible, grow a grass rotation crop to help degrade leftover host material and reduce pathogen levels, and offset the new trees from the previous tree spacing to minimize contact of healthy new roots with any infested roots and soil that may remain. Before placing the tree in the planting hole, line the hole with clean soil and then fill the hole with clean soil at planting. Keeping the crown area dry may help reduce disease severity.
UC IPM Pest Management Guidelines: Walnut