Control of soil and seed borne diseases.

Playing catch-up

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New and aggressive dickeya pathogen has US potato industry searching for answers…

Article by Melanie App – Spudman magazine, AmericaDickeya dianthicola

In 2015, Dickeya dianthicola, a new blackleg pathogen, emerged in the eastern United States, causing significant yield loss. Dickeya is an aggressive pathogen that can travel long distances. Since first emerging, the pathogen has spread to potato crops in the mid-Atlantic region, and has also been detected in Pennsylvania, Maryland, Virginia, North Carolina, West Virginia and Florida. Plant pathologists, agronomists and researchers are scrambling to control the pathogen.

According to Gary Secor, a plant pathologist at North Dakota State University, there’s not much growers can do to control dickeya.

“We don’t have any good chemicals that we can use to manage dickeya,” he said in a recent interview.  Furthermore, there are no antibiotics registered for use in dickeya, which leaves growers powerless to do much but disinfect.

Secor believes that the bacteria spread during handling – cutting, loading and harvest – which means there’s a good chance for the bacteria to contaminate other potatoes. In order for potatoes to become infected, though, the bacteria need a wound to enter. For this reason, careful handling to avoid injury is best.

So far, said Secor, there are no known varieties that are resistant to dickeya. “It’s simply too new,” he said.  He recommends planting clean seed. Since the bacteria can’t survive long in the soil, he also recommends a one-year crop rotation, which should help get rid of the problem.


Scientists search for management solutions 

Meanwhile, on the other side of the country at the University of Maine, Jianjun (Jay) Hao, a researcher in the School of Food and Agriculture, has been busy testing possible controls for dickeya.

He has tested 25 essential oils, including oregano essential oil, copper sulfate, streptomycin (Agri-Mycin from FuFarm) and allyle isothiocyanate, the active ingredient from mustard extract but in this case, synthesized and made commercially available. Streptomycin is a standard antibiotic used in agriculture.

The oregano essential oil worked pretty well, Hao said, and could be effective as a seed treatment, but more research is needed  “If we treat the seeds, it might eliminate cross-contamination,” he said.

Efficacy for copper sulfate was only partial, Hao said. In the lab, in a petri plate test, Hao found that allyle isothiocyanate was equivalent to or better than Agri- Mycin. Tests in the greenhouse are ongoing.

“We can’t tell much yet, but this chemical actually promotes plant growth,” he said. “We don’t know what the mechanism is. It could be eliminating the other minor pathogens or directly enhancing growth. But the treated plants get much stronger,” he said. “Better vigor.”

The next step is to test the allyle isothiocyanate in field trials. Hao said they need at least one growing season before they can make recommendations.

Another part of Hao’s research is looking at how the disease outbreak started. It looks like the outbreak is water-related, he said. “It’s interesting that we got a lot of dickeya-positive samples from water,” he said. “They’re not the same pathogen, but definitely under the same genus. And it looks like it can survive in water.”

This information could be valuable for growers. Hao recommends testing irrigation water in affected areas, especially if growers use a circulated water system that could potentially return the multiplied pathogen to the soil. Samples should be sent to diagnostic labs for testing.


Ongoing Research Expected
Currently, Amy Charkowski, professor and head of bioagricultural science and pest management at Colorado State University, is working on a proposal for a Specialty Crop Research Initiative grant through USDA. Grants are awarded to multi-state projects that support industry needs.

“As far as I know, there’s never been a national effort in the U.S. to work on blackleg,” Charkowski said. “I’m excited about it because this will be the first time we’ve been able to work effectively across state lines on this problem.”

If approved, the project will have four objectives. First, they’d like to improve detection methods. “We’ve already made some progress on that,” she said. The second goal is to better understand resistance and susceptibility.

“We know that we have wild potatoes that are resistant, and we know that cultivated varieties varying their tolerance,” she said. “We’re looking at what’s different between the resistant and the susceptible lines to see if we can identify compounds that could be good breeding targets.”

Another objective is to understand the epidemiology of the disease. “There’s been just a tremendous amount of work in Europe on this, so we’ve got a good starting point, but our climate’s different,” Charkowski said. “It’s a lot more variable than what you’d find in the Netherlands or Scotland. We need additional data for the U.S.”

Part of the challenge is that seed lots can test positive for the pathogen, but not see the disease the next year. In fact, unless there’s a really high incidence of the bacteria, it’s not likely that growers will see the disease at all.

“That makes the seed growers really reluctant to discard a lot, because there might be a 90 percent chance they even see the disease the next year,” Charkowski said. “We need the epidemiological data to try to figure out how to interpret the test results.”

The final goal of the research project is to better understand the economics of both the disease and its management. Chris McIntosh, an economist from the University of Idaho, will be working on this part of the solution.

“We can’t make very good recommendations right now to farmers, but we’re hoping by the end of the project we will be able to,” Charkowski said. “Should this be funded, we’re hoping that by the end of the four years we’ll be able to give growers really good recommendations.”

In the meantime, Charkowski said they’re extremely grateful for the expertise from their European colleagues, Gerry Saddler and Ian Toth in Scotland, Jan van der Wolf in the Netherlands, and   Minna Pirhonen in Finland.

[Reproduced with permission from Spudman magazine – April 2017]