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Potato foliage is more susceptible to late blight strains US-8 and US-24 than to US-23. (Photo: Rick Peters)

Strain Tracking

Information on shifting late blight strains is helping to control this devastating disease.

Through a national initiative, Canadian researchers have found that late blight strains have shifted dramatically across the country since 2009. The researchers have also identified important differences among the strains in such characteristics as fungicide sensitivity, host preferences, and aggressiveness on potato tubers and foliage.

And their findings are making a vital difference in late blight prevention and management.

The research is led by Rick Peters, a research scientist with Agriculture and Agri-Food Canada (AAFC) in Charlottetown, P.E.I., and Larry Kawchuk, an AAFC research scientist in Lethbridge, Alta.

Peters explains that late blight is caused by Phytophthora infestans, a fungus-like organism called an oomycete. This pathogen has a complex life cycle that includes both asexual and sexual reproduction. In asexual reproduction, each generation is genetically the same as the previous generation, although random mutations can occasionally occur to create new strains. Asexual reproduction results in sporangia (spores) that contribute to rapid spread of the disease during the growing season. The organism must infect living host tissue, such as seed potatoes or potatoes in cull piles, to survive the winter.

Sexual reproduction of the late blight pathogen seems to be rare in North America. It requires that the pathogen’s two mating types, called A1 and A2, come in contact with each other. For example, if an area has both the late blight strain US-23, which is an A1 type, and the strain US-8, an A2 type, then sexual recombination might occur. Peters explains that sexual recombination is a concern for two reasons: it increases the possibility of producing new strains, some of which might turn out to be very aggressive; and it produces survival structures called oospores, which can survive in the soil without a living host.

Peters and Kawchuk’s strain research is currently funded under AAFC’s AgriInnovation Program in partnership with the Canadian Horticultural Council and multiple industry partners. Each year, the researchers collect samples of infected potato and tomato tissues in Canada’s potato growing regions. They isolate the pathogen from each sample and determine the isolate’s strain, mating type and sensitivity to Ridomil (metalaxyl), a fungicide used to manage late blight.

Over the past few decades, late blight strains have gone through a number of changes. In the early 1990s, US-1 was the predominant strain in Canada, but by 1996 US-8 had become the main strain in every province.

In 2009, the incidence of late blight in Canada began to increase, and Peters and Kawchuk started tracking the strains across the country. Over the next few years, they found several new strains, such as US-22 (A2 mating type), US-23 (A1) and US-24 (A1). Initially strain diversity increased as both older strains, including US-8 (A2) and US-11 (A1), and new strains occurred in different regions. However, by 2015 and 2016, US-23 had become the predominant strain.

Peters says, “We have seen a very rapid and quite dramatic shift from US-8, which used to be our most common strain about a decade ago. It has been largely replaced by US-23 in most of the country, although we do have some variations. For instance, in 2016, we found some US-8 in B.C., and some US-24 in Quebec.”

Shifts from one predominant strain to another in a region can occur very quickly, even from one growing season to the next. Peters explains, “Late blight doesn’t survive over the winter unless it’s in living tissue like a seed tuber; it doesn’t survive in the soil yet here. So every winter there is a natural halt to disease progression, and then the next spring and summer whatever strain is present earliest in the season is usually the strain that tends to take over. So, for instance, if US-23 is present early because it’s coming in on tomato transplants or potato seed, then that strain gets going early in the season and spreads.”

Peters and Kawchuk’s research also includes testing of the different strains on various hosts. “We’re testing a lot of different tomato and potato varieties for their response to the different strains. So far, for tomatoes we see a lot of susceptibility to US-23 and much less so to US-8 and US-24. We’ve often grown tomatoes next to potatoes infected with US-8, and the tomatoes would hardly be affected. But that has completely flipped with US-23,” says Peters.

Fortunately, several late blight-resistant tomato varieties are available. The researchers have found that these varieties – especially those that have two known late blight resistance genes – are very effective against the disease.

“For potatoes, most of the commercial varieties are fairly susceptible [to the current strains]. The tubers of all varieties are susceptible to US-23, US-24 and US-8. Potato foliage is most susceptible to US-8 and US-24. US-23 will cause disease in potato foliage too, but it doesn’t melt down the plants as quickly as US-8,” says Peters.

“I often call US-23 a sneaky strain. Potato growers tend to be able to manage it pretty well on the foliage, but then the tuber rot phase is more challenging because it is so aggressive on the tubers.”

Another difference between US-8 and the new strains is that US-8 is more resistant to Ridomil. “Initially US-23 has some sensitivity to Ridomil, so that product can provide some benefit with one application. But there seems to be a build-up of insensitivity as the season goes on. So if you are going to use that product, we don’t recommend more than one application,” cautions Peters.

In another component of the research, the researchers are now examining how the new strains respond to different temperature and moisture conditions. Peters notes, “When US-8 first came onto the scene and replaced US-1, we saw that US-8 tended to be able to spread the disease more readily under warmer conditions than US-1.”

Determining how the new strains compare with US-8 will allow fine-tuning of the late blight forecasting programs used by potato growers to make fungicide application decisions.

The Tomato Connection

Back in about 2010, Kawchuk and Peters found a link between infected tomato transplants and the increasing incidence of late blight in Canada. After some initial discoveries of infected transplants at retail garden centres, the researchers had people in various provinces looking for other infected transplants at garden centres and nurseries. Diseased transplants were found in several provinces. Strain analysis resulted in the first discovery of US-23 in Canada and indicated that the movement of retail tomato transplants was an early-season source of the pathogen in Canada.

This tomato angle has been extremely important in late blight prevention in the last few years. In P.E.I., AAFC and the P.E.I. Department of Agriculture and Fisheries have been conducting a strong outreach effort – including radio and TV items, articles, posters and brochures – to encourage people in the tomato industry and home gardeners to look for the disease, to destroy and bag any diseased plants, and to grow late blight-resistant tomato varieties. AAFC has purchased thousands of packages of resistant tomato seed, and Peters and his team have been distributing the seed for free at tomato industry and grower meetings, garden club meetings and so on, to get the word out in P.E.I.

“Many of the nurseries and stores have come on board and are now offering the resistant varieties in both seed and transplant forms,” says Peters.

P.E.I. is not the only region using this type of strategy. For example, Peters has recently talked about the tomato connection at meetings in Manitoba, after that province had a severe late blight outbreak in 2016. And he has sent a big supply of resistant tomato seeds to Manitoba to help in their outreach efforts.

“I think our outreach program is having a significant impact because we really haven’t had late blight here in P.E.I. for a couple of years now,” he notes. “It is going a long way to prevent the start of the disease, which is always hugely critical with late blight. The longer you can keep it from coming into your crop the better off you are.”

Ryan Barrett sees a lot of value in Peters and Kawchuk’s late blight strain research. Barrett is the research coordinator for the P.E.I. Potato Board, one of the grower groups supporting this research.

“It’s important to continue monitoring late blight strains in Canada on an ongoing basis, as work by Rick Peters and others has shown that the influence and prevalence of strains changes over time. Knowing what strain we are dealing with as well as how a strain may differently affect potatoes than previous strains is essential in planning management strategies,” says Barrett.

“For example, the currently dominant US-23 strain is very damaging to tomato plants, and we have been seeing it show up in tomato plants imported into P.E.I. in recent years. Work by Rick Peters and others to highlight this and work with greenhouses and garden retailers to promote healthy and/or blight resistant tomato plants has undoubtedly contributed to lower late blight pressure in P.E.I. in the last couple of seasons.”

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