Inhibiting slimy substance may be link to unraveling devastating apple tree disease

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biofilms play a necessary part in the development of fire blight. (view larger image)

A slimy substance similar to the plaque that contributes to tooth decay plays a critical role in the most deadly apple tree bacterial disease, said Michigan State University (MSU) AgBioResearch scientist George Sundin.

Sundin said biofilms – a mass of bacterial cells embedded in a slime matrix – are necessary for the development of fire blight, a highly contagious bacterial plant disease that can kill entire blocks of apple trees at a time.  Discovering a way to halt biofilm formation in the apple shoot could mean an end to the disease.

“We’re focused on biofilms because they’re absolutely essential to fire blight,” said Sundin, an Extension specialist and professor in the MSU Department of Plant Pathology. “We didn’t want to target something that only contributed to the disease. The bacterium that causes fire blight needs biofilm for two processes: to invade the xylem, the tubes that conduct water in the plant, and to move through those tubes.”

Formation of biofilms eventually plugs the water-conducting tubes, causing the wilting of tree leaves – a defining characteristic of the shoot blight stage of the disease.

Christopher Waters, assistant professor in the MSU Department of Microbiology and Molecular Genetics, has discovered a chemical compound known as ABC-1 (anti-bacterial compounds 1) that inhibits biofilm formation of several bacterial pathogens in the lab.  Restricted to a single state as opposed to the matrix configuration, bacteria are much easier to treat.

“We’ve found that the compound ABC-1 successfully inhibits biofilm formation,” Sundin said.  “We’re very optimistic about this because we don’t think the bacterium can readily overcome a chemical that targets something it essentially needs to cause disease.”

Next the researchers plan to test the compound on apple trees to see if it inhibits the formation of biofilm in the plant.  If it does, Sundin said they will work to determine exactly how growers can utilize ABC-1 in the field. Researchers are also examining a compound called Agilyte, produced by algae in the ocean, Sundin added.

“[Agilyte] is a different kind of biofilm inhibitor than ABC-1,” he said. “This one has already been tested by a colleague of mine in North Carolina for a different bacterial disease and it showed some positive results.”

Fire blight is a serious concern for the Michigan apple industry. The nation’s third largest apple producer, Michigan experienced fire blight epidemics in 1998 and 2000. In 2000, more than 400,000 apple trees in Michigan were killed by the disease, Sundin said. Spray applications of the antibiotic streptomycin have been the most effective management practice, but the bacterium have built up a resistance to the drug.

A new agricultural antibiotic, Kasumin, was applied by growers across the state this year when weather conditions strongly favored fire blight.  A fire blight outbreak would have been especially devastating for Michigan this year—growers are projecting a bumper crop of 26.1 million bushels, nearly 50 percent larger than the 2010 harvest.

“The EPA granted a Section 18 – a specific exemption – so Michigan growers were able to use Kasumin in orchards where there was streptomycin resistance,” Sundin said. “That worked extremely well and prevented an epidemic. I’m optimistic Kasumin can hold us for the next 10 years or more and hopeful that, within that timespan, we’ll come up with some better solutions [such as effective biofilm inhibitors].”

A long-term solution to fire blight is needed in Michigan, where growers produce such popular varieties as Gala, Fuji and Jonathan – all highly susceptible to fire blight.   

“Apples are not being bred for disease resistance—they’re being bred for quality, taste and consumer appeal,” Sundin said. “That’s great, but the downside to that is in regions like the Midwest, where you have lots of rain, apple scab and fire blight problems, it’s especially difficult for growers.”

Sundin and Waters are applying for U.S. Department of Agriculture grant funds that would enable them to continue studying the role that biofilm formation plays in fire blight.

“We’re optimistic that work with ABC-1 and other biofilm-inhibitory compounds will lead to compounds that are ultimately utilized commercially in plant health management,” Sundin said. “Also, there are groups in human medicine that are targeting biofilms in human pathogens. We figure the chances are high that there will be research in human medicine coming out that might be applied to our system. In turn, we hope that our ABC-1 findings may be applicable to many other systems as well.”

In addition to AgBioResearch support, Sundin’s work is funded by Project GREEEN (Generating Research and Extension to meet Economic and Environmental Needs), Michigan’s plant agriculture initiative at MSU.

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