Researchers developing tools to track harmful bacteria

A leafy gall caused by the rhodococcus bacteria grows at the base of a butterfly bush.

The change can happen overnight, a mutation at the microscopic level, causing otherwise healthy ornamentals such as daisies and chrysanthemums to sprout damaging growth defects.

Rhodococcus, a type of soil-borne bacteria, is to blame for the deformity, infecting mostly herbaceous perennial plants with leafy galls, a disease that burdens Oregon’s $900 million greenhouse and nursery industry.

But not all strains of rhodococcus are harmful, according to researchers with Oregon State University. In fact, some may be beneficial, helping to grow more root hairs and improve uptake of water and nutrients.

The transformation of rhodococcus from good to bad lies in tiny DNA molecules known as plasmids, according to OSU. Researchers are now developing tools that will allow nurseries to identify bad rhodococcus before it spreads, avoiding potentially serious losses.

Jeff Chang, associate professor for OSU in the Department of Botany and Plant Pathology, led a recent study published Dec. 12 in the journal eLife. He said plasmids are the switch that turns rhodococcus from beneficial to completely pathogenic, morphing from Dr. Jekyll into Mr. Hyde.

“You can see how very subtle changes in a bacterial genome can change it from being benign to pathogenic,” Chang said. “The ease to which this transition occurs is rather unusual, and it presents a difficult challenge for nurseries.”

Nurseries are the leader in Oregon agriculture production, with most sales going out of state. Not only are ornamental plants produced for aesthetics and beauty, but they also need to be free of disease before they ship out, Chang said. Otherwise, they must be destroyed.

It was Alexandra Weisberg, a postdoctoral fellow in Chang’s lab, who observed plasmids moving between strains of rhodococcus. Plasmids are molecules within bacterial DNA that replicate independently of the cell cycle, giving bacteria new advantages in their environment.

Chang said the intensive hands-on management of plants in nurseries may encourage the rhodococcus bacteria to get together and transfer the disease-causing plasmid.

“So obviously, the key is to keep the nursery site as clean as possible,” he said. “They could have rhodococcus as a persistent pathogen-causing organism on their site.”

Vigilance is crucial, Chang said, since there is no spray known to control rhodococcus. OSU is working to develop a test kit, he added, that will help nursery managers distinguish between the good and bad bacteria and avoid destroying non-infected plants.

The product is still under development, though Chang described it as resembling a pregnancy stick test.

“The hard part is extracting DNA from the bacteria,” he said.

Melodie Putnam, director and chief diagnostician at the OSU Plant Clinic, said the difference between benign and harmful rhodococcus has led to some misdiagnoses of pistachio trees, which were submitted to the university in 2014.

The trees were short and bushy, had knobby stems and would not properly graft, Putnam said. After testing, however, she found only non-pathogenic rhodococcus in the samples.

She hopes the university’s study and testing tools will help growers to accurately identify and diagnose rhodococcus themselves.

“Education is the primary thing that I’m hoping for,” Putnam said.

The project may also lead to better management tools for the harmful strains of rhodococcus, she added.

“Right now, there aren’t any good options,” she said.

Chang said his lab has pre-applied to the USDA for additional funding to continue the research, and possibly figure out what conditions cause plasmids to transfer in the first place.

“This is a huge problem in the nursery industry, which produces plants for their beauty and aesthetic values,” he said.

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