See:
Crown Gall Disease of Nursery Crops
Cause: Crown gall is a tumor-forming disease of plants caused by tumorigenic agrobacteria, many of which are thought to be present in most agricultural soils. Although the crown gall bacterium has a wide host range, plants more likely to have crown gall include all stone and pome fruit, caneberries (such as blackberry and raspberry), euonymus, photinia, poplar, rose, walnut, and willow. Wounds are required for the pathogen to infect a plant where it transfers part of its DNA into the DNA of the plant.
In the Pacific Northwest, Malling 7 is considered the most susceptible apple rootstock to Rhizobium rhizogenes (formerly Agrobacterium tumefaciens), followed by Malling 9 and 26.
The Mark rootstock, while susceptible to Rhizobium, has another gall-like problem that resembles crown gall. The cause of this other gall is unknown but appears to be genetically inherited in this particular rootstock.
Symptoms On young nursery trees, soft, spongy, or wart-like galls develop on the root crown or on pruning cuts to roots. Gall size on mature trees ranges from a fraction of an inch to several inches across. Galls become hard with a rough, fissured surface as they age. Gall tissues are irregular and have no definite growth pattern. If galls completely encircle the trunk of a young tree, it may be girdled and die.
Cultural control
- Discard diseased plants as soon as noticed to avoid cross-contamination.
- Use good sanitation in handling planting stock.
- Minimize wounding and disinfect pruning tools between plants.
- Plant in well-drained soil when soil is below 50°F.
- Avoid fields with a recent history of high crown gall infestation or root-attacking insects and nematodes.
- Rotate susceptible crops with small grains. No registered chemicals are effective.
Chemical control
- Gallex (ready to use) painted on very young galls to reduce further development. Galls may return the next year or, if treated late, may continue to develop. Tissue surrounding the gall 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.
Biological control Crown gall on apple has been much more difficult to control than on Prunus species. Agrobacterium radiobacter K84 has been largely ineffective against apple crown gall unless disease pressure is low.
References Sutton, T.B., Aldwinkle, H.S., Agnello, A.M., and Walgenbach, J.F. 2014. Compendium of Apple and Pear Diseases and Pests, 2nd ed. St. Paul, MN: APS Press.
Warner, G. 1993. Once promising Mark rootstock failing many. Good Fruit Grower 44 (15):21-24.