Crown rust comes after oats

Part of the management challenge comes from its sources and its choice as a cover crop

In a production agriculture world where corn and soybeans have dominated the scene for much of the last 25 years, oats are supposedly one of the forgotten crops. There have been attempts to revive the grain’s standing, including the formation of the Oat and Barley Council of Ontario back in the early 2000s, as well as consumer-based promotions of the crop’s value as a food grain.

Nor can it be denied that oat acres are volatile in Ontario. Although 90,000 acres were harvested in 2001 and 50,000 acres in 2016, the 2015 crop was much larger in the wake of poor planting conditions for wheat in late 2014. But then there was the usual boomerang: the big 2015 crop caused a lower price offering for 2016 and subsequently fewer acres.

In Quebec, oats have been a long-standing leader among cereals, with 259,350 acres harvested in 2010. Although that number dropped to 185,250 in 2016, this was the first year in the past six where oats didn’t outrank all other cereals (wheat eclipsed oats in 2016, with 214,890 acres harvested).

Yet something interesting is also happening with oats. It’s gaining popularity as a cover crop.

According to Dr. Weikai Yan, a research scientist with Agriculture and Agri-Food Canada’s (AAFC) Ottawa Research and Development Centre, oats may be a small cereal crop nationwide, but it’s one of the largest cereal crops in Quebec and New Brunswick. Part of that is attributed to its natural resistance to fusarium head blight (FHB) which can render wheat or barley grains unusable as feed. Yan adds that oats can also be a key rotational component in cropping systems.

“Its values are not only economic but also environmental,” he says, referring to oats’ emerging preference as a cover crop. “It’s becoming a mainstream practice. There are roughly one million acres of winter wheat in Ontario, and it’s estimated (by Peter Johnson, an independent agronomist) that 50 per cent of the land grown to winter wheat is planted with oats as a cover crop after wheat harvest. So the acreage of oats is much higher than we think.”

Yan considers crown rust to be one of the worst crop diseases given that its strains can change or mutate quickly to overcome resistant genes. And he’s not alone in that assessment. He explains that an oats cultivar is resistant to crown rust only if it has a resistance gene that has not been overcome by crown rust strains in a particular region. The problem is that a resistance gene in oats will be overcome by the disease; it’s only a matter of time.

A side-by-side test plot featuring a cultivar with resistance (l) and one without (r).
photo: Courtesy Dr. Weikai Yan, AAFC – Ottawa

What adds to the challenge, aside from the availability and performance of resistance genes, is the transmission of pathogens and the infrequent nature of disease occurrences. Yan points out that there is also no program to develop new resistance genes in oats. That work was carried out by Dr. James Chong at AAFC Winnipeg a few years ago, who used to work on introgression of resistance genes from wild oats to cultivated oats. Currently, there are nine resistance (Pc) genes that are still very effective against crown rust in Eastern Canada.

Pc91 is the most common of the nine and was derived (introgressed) from a wild oats cultivar found in the U.S. It’s effective at all stages of plant development and is the resistance source in AAC Bullet, AAC Oaklin, AAC Almonte and AAC Roskens. Leggett is a cultivar carrying Pc94. Some new cultivars carrying Pc94 are being developed in Eastern Canada. Pc45, Pc50, Pc51, Pc54, Pc58, Pc59, Pc91, Pc94 and Pc96 are still very effective, but Pc38, Pc48, Pc56 and Pc68, all of which were introduced in the 1970s, are no longer effective against crown rust.

“Pc91 is still effective in Eastern Canada but it has already been overcome in the U.S. and some parts of Western Canada,” says Yan, emphasizing the importance of searching for new sources of resistance. “Another strategy we use is to pyramid resistance genes in a single cultivar to slow down the appearance of new virulence genes in crown rust. Pc59, Pc61, Pc91 and Pc94 are effective resistance genes in Eastern Canada, and we’re trying to put these and some others into a single cultivar to achieve lasting resistance.”

Path of pathogens

As for the transmission of crown rust, Dr. Allen Xue, a research scientist in plant pathology with AAFC’s Ottawa Research and Development Centre, says the severity of the disease from year to year depends on environmental and weather conditions. Planting resistant cultivars is the most effective and feasible strategy for reducing the disease’s impact.

“It’s different (in Eastern Canada) than on the Prairies,” says Xue. Here, spores can blow up from the southern U.S., or the primary inoculum can come from the over-wintering sexual cycle on buckthorn. Together this results in a more diverse race population in Ontario and Quebec than in the West.

Xue and Yan have worked together on crown rust since 2013 under the federal government’s Growing Forward 2 initiative, with Xue monitoring the predominant races in Eastern Canada and identifying effective resistance genes for breeding efforts. Crown rust is documented as having the ability to cause yield losses of up to 40 per cent due to damage to the leaves, although Xue believes that number to be closer to 10 per cent on average, based on annual disease surveys in Ontario in the past 10 years.

“In a disease epidemic year, we still see little or no disease in fields with resistant cultivars or those sprayed with fungicides, and severely affected fields with yield losses up to 60 per cent with susceptible cultivars without spraying,” Xue says.

Oats as a cover

Where oats are making a surprising resurgence is as a cover crop. Red clover remains a traditional source, and there are more growers testing cover blends, some with 10 or more species. Quentin Martin, who operates Cribit Seeds, near Winterbourne, Ont., is a big believer in oats’ potential, acknowledging that as a spring cereal crop it will occasionally surpass 100,000 acres. But it’s as a cover crop that can be planted after every wheat, barley or oats acre that misses a red clover seeding where it shows its true upside, which can reach a million acres in the province some years.

Martin agrees that crown rust is still the key disease and notes that AAC Bullet is the best variety as a spring-planted grain. In 2016, he sampled numerous fields and submitted samples to the University of Guelph’s diagnostics lab.

“You’ll see varied results in the fall, and we’ve promoted the use of certified AAC Bullet to improve those odds,” Martin says.

Martin has also noticed a difference between spring and summer environments as they relate to an oats cover crop becoming a new host, similar to buckthorn. He’s had discussions with participants of an Ontario Cereal Crop Committee meeting, and the consensus was that the summer environment is different enough that growers using oats as a cover aren’t “overexposing” it and creating that new host.

The use of oats as a cover crop is an added concern for Yan as well. He believes it makes the disease more frequent, yet in his experience, it’s the wind-blown spores that are the more important source. Rainfall in May and June are key to creating the conditions for disease spores to germinate and develop on oats.

Like Martin, Yan along with Xue and Dr. Baoluo Ma, an agronomist with AAFC Ottawa, performed cover crop variety tests in the fall of 2016. He found cultivars that yielded the most biomass have two things in common: early development (where they’re photoperiod insensitive) and resistance to crown rust. In addition, some cultivars that are normally susceptible to crown rust can be resistant in the fall, which means the crown rust strains can be different from those in the spring.

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Ralph Pearce

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