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Scientist getting to know deadly plant pathogen
By Brad Haire
University of Georgia

As far as plant enemies go, it's one of the world's toughest. A scientist in Athens, Ga., wants to know how and why it's so deadly efficient, in hopes of finding ways to control it in the future.

Ralstonia solanacearum is a bacterial pathogen that attacks a plant from within, destroying it, said Tim Denny, a research plant pathologist with the University of Georgia College of Agricultural and Environmental Sciences.

Global enemy

Most bacterial pathogens are problems to a specific region. But this one causes damage worldwide.

"Part of the reason this pathogen is found all over the globe is that it has an exceptionally wide host range," he said.

There are many different strains of Ralstonia salanacearum. They can infect and kill hundreds of plant species, including important agricultural crops, landscape trees, flowers and shrubs.

The bacterium gets into the plant through the roots. It then migrates quickly throughout the plant, eating and reproducing as it goes. This causes the plant to wilt and die.

“Disease is really the byproduct of this organism's desire to thrive,” he said.

In greenhouse experiments, young plants purposely infected with this bacterium die within a week, he said.

New threat

One nasty type Ralstonia solanacearum (race 3, biovar 2) has the United States Department of Agriculture concerned. It recently entered the country in a shipment of geraniums from Kenya. The strain can kill major U.S. farm crops, like potatoes, vegetables and nursery stock. The USDA is quarantining suspect plants and areas to control its spread.

"Once a field has been infested, it's extremely hard to get rid of this pathogen," Denny said.

Like humans, plants "are covered with microorganisms all the time," he said. "But they usually don’t get sick. But this pathogen has learned how to defeat a plant's natural defense processes to use the plant as a food source."

This unusual ability of Ralstonia solanacearum interests Denny.

Know the enemy

Using genetic, microscopic, biochemical and many other research tools, Denny is studying the pathogen in as many ways as he can. He's trying to understand how it lives and interacts with the plants it kills and the ones it can't.

For example, Denny altered some strains of the pathogen to make bacteria flourescent under certain light wavelengths. Using these strains, he can track their progress as they colonize a plant. This is important in understanding how they move so quickly.

There are no cures for the diseases caused by Ralstonia solanacearum. The only effective way to combat it is through crop rotation: farmers plant crops that aren't susceptible to the bacterium. This can reduce but not eliminate it from a field. But it can be impractical and economically hard for farmers to do.

Denny has learned that the bacterium secretes proteins that help it colonize a plant quickly. He believes plants could be genetically engineered to turn off the bacteria's secretion system, stopping it before it makes a meal of the plant.

(Brad Haire is the former news editor with the University of Georgia College of Agricultural and Environmental Sciences.)

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