Killer viruses could help Michigan cherry farmers

Mar 19, 2019
Originally published on March 20, 2019 5:14 pm

Bacterial canker is a devastating tree disease that affects sweet cherry orchards around the country. There is currently no good way to treat it, but some Michigan scientists are trying to harness bacteria-killing viruses to control it.

Growing tiny killers

Between whirring instruments, pipettes, sticky notes, and hunched-over grad students, there’s very little open bench space in George Sundin’s lab at Michigan State University. Amidst all the noise, you might not notice the microscopic armies being prepared for battle. 

Sundin grows bacteria in his lab -- the kind that cause fruit tree diseases.

He also grows viruses that have evolved specifically to kill them. These are known as bacteriophage, or phage for short.

“They look like science-fiction type creatures. They have kind of a head on them, and then they've got feet that look like a tripod," he says. "Those feet attach specifically to the particular bacterium they're gonna infect.”

After they attach to bacterial cells, they inject their DNA inside and make a bunch of copies of it.

Then they basically blow it up.

“They produce a last protein that pokes a hole into the bacterial cell, kills it, and all these phage are released," says Sundin.

The phage are brutal, but they could help Michigan farmers. 

A cherry farmer's worries

Two-hundred miles away on the Leelanau Peninsula, Jim Bardenhagen sits at his kitchen table talking about all the things cherry farmers worry about: climate change, foreign fruit imports, invasive insects, and tree diseases.

One of the most devastating is bacterial cherry canker. It causes oozing, infected areas and can kill blossoms, branches and trees - it can even take out entire orchards.

“In the spring if we get a wet, cold day during blossom time, it's usually a very serious infection of them," says Bardenhagen. "We’ve had times when we really got hit hard. It takes two or three years to pull out of it, to get production back after that happens.”

Bacterial canker affects every sweet cherry orchard in Michigan. Popular cherry varieties have little natural resistance. The disease can be especially devastating in combination with cold spring temperatures. The bacteria actually make it easier for frost to form on trees.

Worst of all, there isn’t a good treatment available. Farmers sometimes spray a solution containing copper, which is a natural antimicrobial, but the effects on cherry trees are minimal and inconsistent.

Bardenhagen says farmers can spray copper if it makes them feel better, but they don't expect much of a change.

“Other than copper, can't really do much control, other than pruning it out in the winter when we’re pruning," he says.

Find them, grow them

That’s where those bacteria-killing viruses (or phage) in George Sundin’s lab come in. Phage that kill bacterial cherry canker occur naturally in orchards all over Michigan, but in small numbers.

Right now, Sundin’s team is out collecting them. The ground beneath cherry trees is a good place to sample - the phage fall out of the trees and are protected by the soil. He already has some in his fridge.

“We'll try to separate the phage from the soil particles, and then we can purify that and start studying it," he says.  

They will then grow big phage populations that can be sprayed on trees. Sundin says the spray will be safe because the phage can only attack cherry canker bacteria, and won’t infect anything else. But there are a few kinks to work out.

The cherry canker bacteria could become resistant to the phage.

“We're trying to come up with a mixture of about five or six different phage that we can use in one spray solution," Sundin says. "Even if the bacteria became resistant to one of the phage, we’d have five other ones in the mix that could infect it.”

The phage also have to stay on the tree to be effective.

“Phage are very susceptible to UV from sunlight, so you spray phage on the tree, and if it's a sunny day, they're gonna get degraded very quickly," he says.

Sundin and his team are working on using materials like carrot juice to act as a "sunscreen."

“It's gotta be cheap, and it has to be something that you can put in a spray tank, which is 100 gallons,” Sundin says.

Jim Bardenhagen is excited about the treatment, but patient. He knows the lab is different than the orchard.

“It's really good that [Sundin] is working on this," Bardenhagen says. "Before we do a lot with it on the grower end, we need to know more about it and how it's gonna work.”

The phage treatment for bacterial cherry canker is probably five-to-ten years from being ready to use. In the meantime, farmers will keep pruning their cherry trees and hoping for the best.

This story also appeared in the Interlochen Public Radio show Points North. Listen to the entire episode here.

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