This bacteria can set your garden back two years
Q: Last season I ordered several dahlia tubers from Holland. Unfortunately, five rotted in the field before sending up stems. I thought it was just a fluke, but when I went to dig and divide my dahlias several of the tubers from Holland that made it through the season and produced were infested with crown gall. I called the nursery to get answers and they told me most of their dahlia tubers from last year were infected and people from all over the globe had the same issue. The nursery refunded my money from last year’s order but told me that I can’t plant in that plot for two years, as the bacteria that causes the gall will cause more infection no matter what plants I plant next season --- whether from tubers, transplants or seeds. Is that true?
I have a large, well-developed garden plot with water, electricity and deer fencing. If I can’t plant there, that means no veggies to feed my family and no flowers to enjoy.
A: Crown gall is caused by a bacterium called Agrobacterium tumefaciens. The nursery representative is correct that crown gall bacteria can live in the soil for two years. If there are any host weeds, such as bindweed, they can persist much longer.
More than 600 different species of plants can become infected with crown gall, including many common vegetables, weeds, trees and shrubs. It causes the most serious problems for herbaceous plants such as chrysanthemum, dahlia, geranium, marigold, peony and snapdragon by causing knobby swelling on the roots and stems.
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Crown gall can also distort crown growth or cause galls on the bark of woody plants, especially plants in the rose family. These look like tumors.
Crown galls on woody plants are treated by cutting out the galls and using a torch to cauterize the wound left on the crown. This treatment has to be done carefully so as not to damage the tree or set a fire.
To manage the crown gall bacteria in your garden you may want to combine a couple of different treatments.
One control is to grow a green manure crop of grains such as wheat, rye or oats. These crops can reduce the bacteria in the soil but will not completely eliminate the bacteria.
Another strategy is to solarize the soil. This involves covering the affect soil with clear plastic for four to six weeks in summer. If done correctly this can rid the top six inches of soil of the crown gall bacteria, but will also kill all beneficial organisms residing in the soil.
Another control for crown gall is to dip all seeds or roots in a solution of biological control agents K-84 or K-1026 before planting. This can reduce infections by most strains of pathogenic crown gall bacteria, but not the strain that affects grape vines. Nogall is the name of a commercially available biological control agent that contains the K-1026 soil bacterium.
To read more about crown galls, check out the pest notes found on the University of California Integrated Pest Management site at bit.ly/3u2spuR.
The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or email email@example.com. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.