Blot on the Landscape

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[deck]As researchers warn producers across the country not to underestimate the deceptive new strain currently dominating the late blight pathogen pool, Alberta potato growers are learning more about the enemy at their door.[/deck]

In the heart of Alberta’s processing potato country, Jake Schutter is prepared for the worst. Should late blight invade his fields east of Taber this season, he’ll be ready for it. “On our farm, we go with a full arsenal of fungicides. So far, we’ve been spared any storage losses related to late blight. There have been a few farmers who were not so lucky,” he says.

Times have changed for Alberta growers. Up until four years ago, the province was virtually late blight-free. Lately, wet springs and high humidity — uncharacteristic for this region — have nurtured the growth and spread of the disease, causing sticker shock for some growers due to the increased number of fungicide applications needed to protect crops from late blight.

“We went from four sprays to 10 applications of fungicides. It’s a huge increase in cost,” says Schutter. “We have applied a lot more fungicide and more expensive, newer types to be proactive and control the disease.”

All totalled, Alberta growers took a big hit to the pocketbook in 2013. “We estimated about $12.5 million last year in added costs for [crop] protection and shrink in the potato shed,” says Jeremy Carter, technical director of the Potato Growers of Alberta.

Not to mention the toll anxiety about the disease has taken on more than a few growers. “[There’s] the stress factor of dealing with something you’re not familiar with…. Then you have to wonder if it will transfer from the plants above ground into the tubers and, if it does, once you enter storage season you might have breakdown in your bins,” says Schutter. “To stop it from keeping you up at night, you have to do your due diligence.”

This take-charge attitude is echoed throughout the Alberta potato-growing community, says Terence Hochstein, PGA’s executive director. “Alberta growers are very proactive. They have an excellent relationship with each other,” he says. “They’re doing everything they possibly can to minimize the effect [of late blight] and to try to eradicate it. They view it as a community disease.”

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Alberta grower Jake Schutter is prepared to use “a full arsenal of fungicides” to protect his potato fields near Taber from late blight.

Three-Pronged Attack

Offensive operations are also underway at the PGA via a three-pronged assault including spore trapping, a publicity campaign and a speaker series on the issues surrounding late blight. “Alberta has a history of having some of the highest quality processing potatoes in North America, and we want to keep our record at that. So we need to get in front of this. That’s why we’re putting this initiative together, this three-pronged approach with our industry,” says Carter.

Spore traps will detect late blight-causing spores before they infect crops, while a literature blitz focused on the public, including home gardeners, will be mobilized this spring targeting garden centres, box stores and other venues selling seed potatoes and tomato plants. A speaker series will round out the offensive by providing growers with a comprehensive crash course on everything late blight. The first session was held in February and a second will be held in June, with a follow-up scheduled at the PGA annual meeting in November, covering disease triangles, late blight life cycle, different strains throughout North America, crop protection products, among other topics.

Aimed at bringing all growers up to speed on late blight, the speaker series is already in demand, says Hochstein. “They want to look at ways to deal with cull piles, to deal with all of the problems and the spread of the disease — the growers are asking for this.”

Schutter is among those attending the events. He’s eager to learn as much as he can about the enemy. He’s also witnessed the attitude about-face that has taken place among Alberta growers. “A few years back, everybody just shrugged their shoulders — late blight, that’s elsewhere, that’s Manitoba or the East Coast, we don’t have late blight in Alberta,” says Schutter. “The realization has come, especially in the last year — it’s the real thing, and we have to watch, we have to be careful where we buy our seed … and we have to take measures with fungicides.”

However, it’s not only Alberta farmers who will benefit from learning more about late blight this year: even seasoned veterans are being encouraged to take the measure of the latest strain to dominate pathogen populations.

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Under moist conditions, late blight can appear as a white, downy mildew on potato leaves.
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Late blight spots can grow to large brown or black necrotic lesions on leaves and spread via petioles to the stems, eventually killing the entire plant.

A Deceptive Enemy

In 2013, for the second year running, US-23 was the predominant late blight strain across Canada, confirms Khalil Al-Mughrabi, potato pathologist with the Potato Development Centre, New Brunswick Department of Agriculture, Aquaculture and Fisheries. Although pathogen populations can shift, and shift quickly, there’s a good chance this strain will be showing up in farmers’ fields in 2014. Understanding the US-23 strain could save farmers from a nasty surprise at harvest or during the storage season.

US-23 entered the Canadian landscape and farmers’ fields in 2011 and has since displaced other strains. Plants infected by US-23 still present lesions on stems and leaves typically caused by the late blight pathogen, Phytophthora infestans, however, researchers warn the characteristics of this particular strain are different from older strains.

“It’s a sneaky strain in potatoes because you tend not to get as aggressive disease in the leaves and stems like you did with US-8. Growers will notice it, but they won’t notice it moving as quickly as the older strain did,” says Rick Peters, a research scientist with Agriculture and Agri-Food Canada.

But don’t be fooled by less damage to above-ground plants when scouting fields. “Unfortunately, what we’re finding is it’s very aggressive on the tubers. Even in situations where we’ve had fields with very low incidence of symptoms on leaves and stems, there’s pretty high incidence in the tubers because of the spores landing on the soil and washing in with the rain,” says Peters. “On the surface it looks not too bad, but suddenly at harvest there’s major tuber rot because of its aggressiveness on tubers.”

Another difference between US-23 and US-8 — the predominant strain from the mid-90s to around 2010 — is its effect on tomatoes. US-8 was a particularly aggressive strain on potatoes, usually infecting farmers’ fields first before causing issues for surrounding tomato growers, whereas US-23 is considered a tomato-associated strain and is especially aggressive on tomatoes.

“Tomatoes in home gardens get hammered close to the end of summer. Those spores can then infect the surrounding potato-growing areas. It used to be the other way around,” says Peters.

The specific characteristics of US-23 have influenced disease management strategies. For example, more collaboration between industries is needed, says Peters.

“Now that the tomato is a factor, an understanding between those different industries will be important, as well as home gardeners who we didn’t have to think about very much in the past, but now they’re part of the picture. We need to educate everybody on this disease and get them involved in the management of it,” he says.

Disease management in the field in terms of foliar spray programs won’t change all that much, but because infection from this strain may not be as visible and aggressive as older strains, growers should not ease up on protecting their crops, warns Peters.

“We still have to get out there and be cautious in applying those products appropriately — and not waiting — because you may miss some of those early infections that perhaps aren’t as visible as they normally would be with the older strain,” he says. “Standard management still applies to this strain, it’s just tweaked, because its characteristics are a little different. We have to think about tomatoes more than we used to and we have to think about protecting the tubers more now.”

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Potato infected with late blight, which causes the skin to darken and discolouration in the flesh.

Phosphite Fungicides

In terms of chemistries, there are many products available to prevent and manage late blight; however, researchers are singling out one group in particular. “Phosphite fungicides stand out. In recent years, the benefits of phosphites as part of a comprehensive integrated pest management program, particularly to help with tuber blight management, have been promoted,” says Al-Mughrabi.

Phosphites, such as Phostrol and Confine Extra, are systemic fungicides that move within the plant, even down to the tubers, to prevent tuber rot infections in the field, adds Peters. It is important they be used in addition to a regular spray program for late blight.

“Phosphites have the potential to not only help manage the foliar disease but to help with tuber rot, which is particularly important with this new strain…. They also have a good environmental profile,” says Peters.

The word on phosphites is spreading. In Alberta, Schutter is considering adding them to his crop protection toolkit this year. “There are some growers that have been using these products for one or two years, mainly those who were closest to the outbreaks, and they’ve had very good results,” says Schutter. “We’re thinking to go the same route on our farm. That being said, you should not stop with a normal preventative program.”

In the end, Mother Nature will have the final say on what areas are hit and how hard, but changing environmental conditions could mean regions susceptible to late blight will keep shifting. Conventional thinking about the geography of late blight no longer applies. From Alberta with its warm, wet, humid springs to hot, dry summers in Prince Edward Island, weather patterns are certainly deviating from the norm: farmers nationwide must be on alert for late blight this season.

“Stay vigilant and make sure blight management is an important part of your program wherever you are in the country, because with these new strains and the changing climate, there’s no one who’s really safe from blight anymore. Everyone’s got to get up to speed on this disease and be proactive in managing it,” says Peters.

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Coming to a Field Near You

Late blight, caused by Phytophthora infestans, can be spread across continents or regionally. The disease can be transported long distances by the movement of infected potato seed or tomato transplants around the continent or within a country. Wind and rain can also move late blight spores long distances, but this movement is generally considered regional spread.

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New Threat on the Horizon

The possibility of different mating types of Phytophthora infestans recombining to produce new strains is a reality in Canada. There are two main mating types of the late blight pathogen — A1 and A2. For example, the mating type of US-23 is A1 while US-8 is A2. If the A1 and A2 types have an opportunity to mate, an oospore is created from the union. From that, a brand-new strain is formed. “So far, it’s a minor issue in North America,” says Rick Peters, a research scientist with Agriculture and Agri-Food Canada. “But it could become a major one if it starts to happen more frequently.” That’s the good news.

The bad news is recombination could mean new strains that are more aggressive, with characteristics growers and researchers haven’t encountered yet. And the oospores could potentially survive in the soil and cause late blight earlier than growers have experienced in the past. “That’s already happened in some places in Scandinavia and Europe,” says Peters. “It’s something we’re trying to watch for and keep a handle on.”

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Late Blight Scout: What to Look For

  • Small, irregularly shaped spots from pale to dark green in colour that appear water-soaked on leaves and stems.
  • Under favourable environmental conditions, spots grow to large, brown- or purple/black-coloured necrotic lesions, which may kill entire leaflets and spread via petioles to the stem, eventually killing the entire plant.
  • Under moist conditions a white, downy mildew appears on the leaves, mostly on the undersides.

Source: “Understanding and Managing Potato Late Blight,” by Khalil Al-Mughrabi, potato pathologist with the Potato Development Centre, New Brunswick Department of Agriculture, Aquaculture and Fisheries.

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