Crown gall is a common disease caused by a bacterium called Agrobacterium tumefaciens. Infected plants are recognized by galls (tumor-like growths) that can be as large as several inches across. They typically form near the soil line, hence the name crown gall. However, they also may form on stems, branches, or under the ground. Rose, euonymus, poplar, apple, and cherry are a few plants that frequently become infected, but just about every other nongrass species is at risk.
Tops of infected plants often look normal, but if there is a serious infection, the galls can interfere with water uptake and nutrient flow. Plants with a severe infection might be stunted, fruit development could be impaired, or leaves may be off color and mimic fertility problems.
A. tumefaciens enters plants through wounds. Once inside the plant, the bacterium reproduces, and then injects a portion of its genetic material (DNA) into the plant. The bacterial DNA directs the plant to make hormones, which cause plant cells to uncontrollably divide and enlarge.
High numbers of bacterial cells live in the outer layer of galls, which turn dark, dry, and typically fall off into the soil as galls age. The bacterium can survive winter either in the soil or in an infected plant.
The best control for crown gall is avoidance. Be sure to inspect plants before buying them. Remember, the bacterium can live in the soil for several years, even without plants. Growing a species that can't be infected by A. tumefaciens, such as grass, for 3 years can help eliminate the bacterium from the soil. Avoid injuring your plants because A. tumefaciens needs an opening to get in. Control insects that might feed on lower stems, crowns, or roots. Pruning out galls is an option if they're not on the main stem or below the ground. Take care to clean tools with a 10% bleach solution before use on healthy plants or plant parts. A biological control treatment is available commercially, as well. It's actually another species of Agrobacterium called A. radiobacter that produces an antibiotic that works against A. tumefaciens.
This article originally appeared in the February 21, 2003 issue, p. 16.