AgronomyOutsmarting Potato Wart

Outsmarting Potato Wart

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New resistant varieties are an important tool for managing and potentially eradicating potato wart. However, they are just one layer of protection needed to control the disease and sustain trade.

Imagine a microscopic intruder lying in wait for decades, ready to spring into action when its target comes close. That’s the case for potato wart: a resilient, persistent and — as P.E.I. farmers know too well — economically devastating disease. Scientists are striving to outsmart this unique and adaptable pathogen with resistant varieties, but it’s no easy task.

As most farmers know, diseases require the ‘disease triangle’ — a host, pathogen and suitable environment — to successfully spread infection. Potato wart spores are unique in that they can remain in a resting state for decades, not just until the environment is right (as many disease spores do), but until a host is in close proximity. When the spores receive chemical messages confirming a potato plant is nearby, the spores may ‘wake up’, releasing zoospores that ‘swim’ to the plant.

When a resistant potato variety is in the vicinity and the spore is activated, the variety’s resistant genes prevent the spores from infecting the potato tubers. With no host, the activated spores die. Given that unique mechanism, could planting resistant varieties essentially starve out the potato wart organism, sanitizing a field? Not quite or, more specifically, not now.

David De Koeyer, a potato breeder and geneticist who leads Agriculture and Agri-Food Canada’s National Potato Breeding Program in Fredericton, New Brunswick says the concept works in theory but isn’t reliable in practice, at least not yet in tight rotation fields. That’s because not all resting spores activate at the same time.

“Potato wart spores can remain dormant for a long period of time,” says De Koeyer. “The problem is they’re not all going to germinate and turn into zoospores at the same time. What triggers germination of the spores? We don’t know, but if we could understand that better, that could help us develop better resistance or other mechanisms to control the disease.”

There’s another factor at play too: complexity, both of the pathogen itself and the spectrum of resistance. Currently more than 40 pathotypes of the potato wart pathogen have been discovered around the world. Three — pathotypes 2, 6 and 8 — have been found in Newfoundland and Labrador, while two — pathotypes 6 and 8 — have been found on P.E.I. Worldwide, there are likely more pathotypes that haven’t yet been characterized, and certainly more still that will develop as the disease adapts and changes.

“What we know is that the fungus can change. That’s a challenge because if you only rely on a single resistance gene, it dramatically increases the likelihood that those resistance genes can be overcome,” says De Koeyer.

The good news is that, on P.E.I., inoculum levels are extremely low. That’s not the case in other infected regions, however. In Europe, for example, where inoculum levels in some areas can be high, genetic resistance has been shown to be overcome repeatedly.

De Koeyer says resistance is a critical component of the battle against potato wart and could offer a key to eradication of the disease in the future. Half a dozen resistant genes have been characterized to date, leading to multiple resistant varieties available on the market. That said, not all resistance is created equal. Actual levels of resistance vary widely between resistant varieties.

“Ideally, there would be no infection and no development of galls and no spore formation [when a resistant variety is planted], but in fact we know there is a range of responses,” De Koeyer says.

“Some resistant varieties are quite susceptible to some pathotypes or may show a few symptoms; some seem to be very resistant against multiple pathotypes. We’d like to confidently say resistance on its own could starve out the pathogen, but it’s the kind of thing you’d have to study on a very large scale, made more difficult because the actual level of resistance varies widely based on the pathovar and the individual or combination of resistance genes.”

Several research papers1 suggest that using resistant varieties, in concert with natural degradation, may help eliminate potato wart spores over time. De Koeyer isn’t confident enough in resistant varieties’ efficacy to make that recommendation.

For now, he says, “While there aren’t any known ways to eliminate the pathogen, what we can say is that with a highly resistant variety, the population of a pathogen is not going to increase. I would look at resistance more as a protection from further spread rather than an elimination of the disease.”

If resistance can hold infection at bay, could resistance also be used as a preventative treatment?

“From a scientific point of view, and even from recommendations from the International Advisory Panel on Potato Wart, part of the recommendation was that there be more resistant varieties available as a preventative measure,” De Koeyer says.

“As breeders and as the industry, I think we all would like to have more varieties with resistance available for all the different market classes to give growers in the affected areas as well as outside the affected areas options on what they grow.”

AAFC’s efforts in breeding for resistance to potato wart aren’t new. Alongside a long list of other production and agronomic priorities, AAFC’s potato breeding program has included potato wart resistance for some of its breeding lines for decades. Those efforts picked up significantly after potato wart was discovered on PEI in 2000.

 “When we were breeding to find potato wart resistant varieties for Newfoundland, it was a relatively niche effort,” De Koeyer says. “However, when the latest detections of potato wart occurred on PEI, we reevaluated our efforts and decided we needed to up our game to keep ahead of the pest.”

In the early days of Canada’s potato wart resistance efforts, breeders sought individual resistant genes. Now, their ultimate goal is to develop resistance that has more longevity through multiple gene resistance. To get there, AAFC is enhancing its facilities and quarantine research fields required to confidently evaluate resistance to potato wart. Surprisingly, that’s more of a challenge than one might think.

“It’s not always easy to have the perfect environmental conditions that promote the disease development in order to study it in an experimental setting,” says De Koeyer.

While AAFC has increased its capacity to do potato wart resistance breeding research, De Koeyer says we’re still in what he calls the ‘beginning phases’.

“We’re increasing capacity and increasing expertise to be able to quickly screen enough material to validate our markers. Right now, we have a few genetic markers that we’re working with, but we don’t have enough data to confirm in which scenarios these markers do or don’t work, and how predictive these markers really are.”

He says some of them look like they have good potential, but finding effectively resistant genes is complicated by potatoes’ diversity. The more genetically unrelated the potatoes are from each other, the less likely it is that any specific marker is going to work.

From a global perspective, Europe has dealt with this disease for much longer with much higher inoculum levels than P.E.I., although potato wart has been known to be present in Newfoundland for over 100 years. AAFC have partnered with researchers in Europe since the disease was detected in P.E.I. in 2000. A lot of the efforts following this detection were on diagnostics — specifically, being able to detect the disease in soil — but more recent efforts have focused on resistant varieties.

“In the last few years, we’ve been strengthening our relationships with European collaborators further and building on what they’ve developed,” says De Koeyer. “The markers we’re working with right now would be an example of that: they were developed in a lab in the Netherlands, now we’re testing them here. In the future, we’ll collaborate with other breeders in North America so they can screen their germplasm with these markers once the predictive ability of the markers is validated using Canadian data, and hopefully we’ll all make progress together.”

There are currently five varieties listed on CFIA’s resistant variety list. Efforts continue to seek out additional varieties beyond our borders that could successfully grow in Canadian growing conditions and may offer resistance to the specific potato wart pathotypes that exist here.

“It’s quite a complicated disease,” says De Koeyer. “There’s lots we don’t understand about it. I think we’re all really excited about the progress that can be made, but this does take time to put together. We’re really hoping that in the next few years we’ll continue to make good progress, that we’ll validate the results we’re working on, and that we’ll put together new tools that make a meaningful difference in the fight against potato wart.”

1 Baayen et al’s Resistance of potato cultivars to Synchytrium endobioticum in field and laboratory tests, risk of secondary infection, and implications for phytosanitary regulations. EPPO Bulletin; and Rintelen et al’s Detection and longevity of potato wart pathogen in once-infested foci Journal of Plant Diseases and Protection

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