Symptoms and Signs
Crown gall appears as rough, abnormal tumors or galls at or below the soil surface on roots, the crown, 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.
Comments on the Disease
Crown gall is caused by a bacterium, 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. Seedling Paradox rootstock is especially susceptible.
Reduce the incidence of crown gall by planting noninfected "clean" trees. For seedling rootstocks, nurseries should collect the seed so it never contacts the soil where the Agrobacterium resides. 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. Strains of Agrobacterium tumefaciens (formerly A. radiobacter) strain K-84 are available as commercial products. However, it is effective only as a preventive treatment and does not eradicate galls. Use as a root dip or spray before heeling in (covering with moist soil until trees are planted) or planting.
Look for and manage 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 or early summer because with rapid tree growth occurring, new callous tissue is formed relatively quickly.
Management 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.
- It is always best to remove galls when they are small. Usually by the time they're seen at ground level, there may be extensive galling on crown area beneath soil.Only treat trees that are vigorous. Stunted trees should be removed.
- If trees less than 4 years old are severely affected with galls, it is more economical to remove the trees and replant.
- Treatment may be effective on older trees. The decision whether to treat galls or remove trees depends on tree vigor, 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. Because no water is used, treatment can be done immediately after removal.
- 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 is advantageous to surgically remove the main part of 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. Flaming should never be used on very young trees.
- 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 and then applying the treatment.
- Leave the treated areas uncovered for the summer and re-treat if galls begin to regrow. Treatment success is about 80%.
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.
- Fumigate with Telone C35.
- Consider rootstock's resistance . Clonal ParadoxRX1 has moderate resistance to crown gall. Clonal Paradox Vlach and VX211 have low resistance to crown gall. Although seedling black rootstock is not as susceptible to crown gall as seedling Paradox, walnut varieties on black rootstock generally aren't as vigorous so should be planted on loamy soils.
- 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.
- Keep the crown area dry to help reduce disease severity.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.|
|(Telone C-35)||Label rates||0||0|
|COMMENTS: The maximum allowable rate of Telone C-35 in California is 46.7 gal/acre, regardless of label rates.|
|B.||AGROBACTERIUM TUMEFACIENS STRAIN K-84|
|(Galltrol A)||Label rates||12||0|
|COMMENTS: Applied at planting to tree roots. This biological control is marketed as Galltrol A, Norbac 84C, Nogall, or Diegall and is a preparation of Agrobacterium tumafaciens strain K-84 (formerly A. radiobacter). It is effective only as a preventative treatment. Effectiveness can be variable on walnut trees. Use as a root dip or spray before heeling in or planting. It does not eradicate galls.|
|COMMENTS: For removal of existing galls, apply to the crown where large galls were physically removed or directly to small galls on roots during winter and spring. After removing the gall from the tree, allow the tissue to dry 2 to 3 days before directly applying Gallex to injury. Overlap treatment to healthy tissue by 1 inch.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.|