Getting to the Bottom of Common Scab in Canada

By

 Ashley Robinson

 ProMED

February 28, 2022

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The first national common scab research project has the Canadian potato industry seeing common scab in a whole new light.

Tubers across Canada have fallen victim to a bacterium lurking in the soil for years. When soils are dry, brown lesions appear on potatoes. And while these spuds can still be eaten safely, the lesions in some cases cut deep holes in them and cause problems for the industry.

While some research has been done in Canada on the potato disease common scab, there hasn’t been much done on a national level. It hasn’t just been one region of the Great White North affected by this disease, growers across the country have found themselves stuck with these ugly spuds.

For Tracy Shinners-Carnelley, vice president of research, quality and sustainability at Peak of the Market in Manitoba, and Newton Yorinori, director of plant breeding, seed development and research at Cavendish Farms in Prince Edward Island, they knew something needed to be done on a national level to address common scab.

At the time, the Canadian Potato Council (CPC) was looking for research projects to focus on. The group was applying for cluster projects funded by the Canadian Agricultural Partnership (CAP) with the Canadian Horticultural Council. Shinners-Carnelley and Yorinori thought a closer look at common scab was a good fit for the cluster projects.

“In Ontario, Prince Edward Island and New Brunswick, they had worked on scab a lot. But we hadn’t and my first thought was well, who do I know that’s been active in this research space? And that’s Claudia,” Shinners-Carnelley explains in a phone interview. “I was trying to find out what was already happening. And start to ask some questions about where we start potentially with some strategies around attempting to manage scab, which is always such a difficult one because there are no easy answers.”

Shinners-Carnelley and Yorinori approached Claudia Goyer, a research scientist at Agriculture and Agri-Food Canada (AAFC)’s Fredericton Research and Development Centre. They wanted to work with her to try and find a way to control the bacteria affecting potatoes.

What is Common Scab?

Common scab has been around for more than 100 years. It’s caused by a filamentous bacterium found in soil. When soil conditions are dry, it enters tubers through the lenticels making brownish lesions on spuds. As the potatoes grow the lesions become larger. As it’s a soil borne disease and a bacterium, Goyer says it’s harder to control as there are no chemicals available to control it.

“Once you have common scab in your fields, it’s really difficult to get rid of it. There’s different species that are causing common scab but the one that is found like pretty much everywhere in the world is Streptomyces scabies,” Goyer explains in a phone interview.

There are other species of common scab though that are found regionally, including Streptomyces acidiscabies and Streptomyces turgidiscabies. According to Goyer they all produce a group of plant toxins called thaxtomins — which is how common scab causes the brown lesions on potatoes.

Goyer notes the lesions aren’t dangerous to humans. Potatoes with common scab can still be consumed, however common scab makes them “ugly.” In the worse cases of common scab, the lesions can go deep making holes in the potatoes.

The industry rule is if more than five per cent of the surface of tubers are covered with common scab, they’re unable to be sold to the table market. Goyer says spuds with common scab are harder to peel, making them less desirable for the fry market also.

“It’s really an issue both for table and processing, then of course it’s even worse for seed production. They really don’t want common scab because nobody wants to spread that disease everywhere,” she adds.

Irrigation does help reduce common scab incidence though as it keeps soils from drying out, Goyer explains.

Searching for a Canadian Solution to Common Scab

In 2018, Goyer started on her common scab project. Working with collaborators in Manitoba, Ontario, P.E.I. and New Brunswick, they collected samples of potatoes with common scab symptoms for testing. Pathogens of common scab present in Canada were then isolated from infected tubers and characterized using molecular testing. So far Goyer says they have a collection of 300 isolates with at least 20 genetic groups.

“This shows that there’s a lot of diversity in the pathogens, which then might explain why we’re having so much trouble finding solutions to control the disease, right? Like it’s so widely different in how they behave, it becomes more difficult to find a control method that works everywhere,” she explains.

Goyer says the most common species found in Canada is Streptomyces scabies. Another species, Streptomyces acidiscabies, was also found to be present, but it’s more common in acidic soils and was first discovered in Maine.

“THIS SHOWS THAT THERE’S A LOT OF DIVERSITY IN THE PATHOGENS, WHICH THEN MIGHT EXPLAIN WHY WE’RE HAVING SO MUCH TROUBLE FINDING SOLUTIONS TO CONTROL THE DISEASE, RIGHT? LIKE IT’S SO WIDELY DIFFERENT IN HOW THEY BEHAVE, IT BECOMES MORE DIFFICULT TO FIND A CONTROL METHOD THAT WORKS EVERYWHERE.” CLAUDIA GOYER

After determining the genetic groups, Goyer and her team started to develop tools to look closer at how they are distributed across Canada. The group is also looking at ways to control common scab in fields.

Goyer says they’ve looked at how growing certain crops before potatoes can help hold soil moisture in to reduce common scab incidence. However, they’ve had trouble establishing the nurse crops. They have also tried beef manure compost, liquid mustard and peroxide based products.

“We also tried different fertilizer products like ammonium sulfate, which is supposed to make the soil more acidic. The common scab pathogen doesn’t grow well when it’s more acidic, so we thought perhaps this would help. And none of these really work,” she adds.

There have been two options which have shown promise though. They tried the biopesticide Serenade Soil in Fredericton and saw good results for several years. In Manitoba, it reduced common scab severity by 40 per cent compared to an untreated control. The best results were seen with an auxin product, 2,4-D, which is basically an herbicide Goyer says. Used in miniscule amounts applied as a fine mist in Manitoba, it reduced common scab severity and produced 69 per cent marketable tubers compared to less than five per cent in the untreated control.

“We’re now evaluating if we need to tweak how much 2,4-D to put in or maybe we need to apply it at a different time depending on the cultivars,” Goyer explains.

The project will complete its final year of trials in 2022 with full results released in 2023.

Common Scab Project Breakdown

• Research team
  • Claudia Goyer with AAFC in Fredericton, N.B.
  • Martin Filion with Université de Moncton in New Brunswick
  • Tracy Shinners-Carnelley with Peak of the Market in Manitoba
  • Newton Yorinori with Cavendish Farms in P.E.I.
  • Rick Peters with AAFC in Charlottetown, P.E.I.
• Provinces where trials are being done:
  • Manitoba
  • New Brunswick
  • Prince Edward Island

Header Photo — Potatoes with common scab symptoms on them. Photo: Claudia Goyer

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