Experts tackle readers’ garden questions.
Q: I have an Ouachita Thornless blackberry bush in a large pot in my backyard. It has this strange woody looking growth at the base of the canes. Is this normal or some sort of parasite? If it is bad, how do I take care of it?
Marilyn Schiveley, Folsom
Bee garden writer Debbie Arrington: That’s a crown gall. It’s like a woody tumor on the crown of the plant, caused by a bacterial infection.
Crown gall is widespread. It can infect apples, grapes, peaches and roses as well as all brambles and many ornamental plants.
According to University of California integrated pest management research, crown gall is caused by the soil-borne bacteria, Agrobacterium tumifaciens. It enters the plant through wounds. Those wounds may be natural (caused by frost, wind or insect damage) or mechanical (such as damage during pruning and weeding around the canes). Often the wounds come during transplanting or handling in the nursery. Wounds also may be caused by sucker removal or use of string trimmers around the plant.
It doesn’t take much of an opening for this bacteria to enter. The bacteria can infect newly emerging roots or growth cracks in canes. Crown gall bacteria can overwinter in the soil. It spreads by splashing rain or running water. It also may be transferred on tools during pruning and cultivation.
Once inside the plant, the bacteria causes distorted growth – or galls – usually around the root crown, where the canes emerge. These galls start out tan and spongy, gradually hardening and darkening as they age.
The gall stresses the plant and can block the flow of nutrients up the canes. It also can interfere with the production of new canes. Plants can become stunted and less resistant to drought stress.
If the berry bush is bearing fruit and seems healthy, it’s OK. Just leave the gall alone. But if the plant is not bearing, then it’s time to get a new plant.
Make sure to change the soil in the pot. Discard the old potting soil; bag it up and put it in the trash (it’s infected with the bacteria). Clean the pot thoroughly with soap and water before refilling with fresh potting soil.
For more tips on crown gall, visit the university’s IPM website at http://ipm.ucanr.edu.
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