Stripe rust, which can affect both wheat and barley crops, generally arrives in Canada on wind currents from the southern U.S. states and the Pacific Northwest. Some research out of Alberta is also showing that inoculum from the disease is able to overwinter and may be contributing to more severe outbreaks in that region.
Also known as yellow rust, this disease has been present, in varying degrees, across the Prairies for the last 10 to 15 years. Normally, the stripe rust pathogen thrives in cooler weather and usually does not survive in hot, dry conditions. Nighttime temperatures around 10 C, with a few hours of moisture on plant leaves, are ideal for the inoculum to germinate and infect the plant, however variants adapted to warmer environmental conditions have also been identified. U.S. research suggests that recent stripe rust epidemics there might have been associated with some wheat stripe rust strains adapted to warmer temperatures. Similar work at Alberta Agriculture &Rural Development’s Research Centre at Lacombe suggests that wheat stripe rust strains can adapt more easily to relatively warmer temperatures than barley stripe rust strains.
One of the biggest factors influencing the severity of a stripe rust outbreak is the time of its arrival. Early arrival of stripe rust inoculum into Canada gives the pathogen more time to spread and have a potential impact on yields, if environmental conditions are favourable.
Stripe rust, if the outbreak is early and severe, can significantly decrease yields. A severe infestation in central Alberta in 2000 reduced yields of some varieties of spring wheat in test plots by 45 per cent. The loss of grain yield is caused by the loss of green leaf area, which affects photosynthesis of the plant and grain fill.
Stripe rust incidences vary greatly across the Prairie provinces. Here is a brief analysis of the stripe rust situation across the Prairies in 2010, and a discussion of the factors in its spread, new research relating to stripe rust and management options.
Wheat stripe rust was detected in June in Manitoba last year, which is fairly early compared to other years, says Manitoba Agriculture, Food and Rural Initiative’s provincial cereals crop specialist, Pam de Rocquigny, but it didn’t reach economic threshold levels in the province.
“It’s an early-season disease that tends to favour cooler conditions for further production of inoculum,” says de Rocquigny. “So we will see wheat stripe rust come up here but often our hot summers will bring things to a halt.”
The stripe rust which ends up in Manitoba originates in the southern U.S. states like Louisiana and Texas; Alberta’s strain of the disease comes from the Pacific Northwest, meaning that the two have a slightly different biology.
“There is a lot of stripe rust every year in the Pacific Northwest and so it’s a matter of whether or not it gets across the mountains to Alberta,” says Tom Fetch, a cereal stem rust research scientist at Agriculture and Agri- Food Canada’s Winnipeg Cereal Research Centre. “In Manitoba and Saskatchewan it’s a lot more variable. Last year we had a lot of stripe rust inoculum in the southern Great Plains and we wanted to make sure our growers knew about it. For 2011, we will wait and see over the next month or two how it develops.”
In its leaf spotting disease survey for 2010, the Saskatchewan Ministry of Agriculture detected trace amounts of stripe rust in 13 per cent of the flag-leaf stage samples it tested, which is slightly higher than usual, says provincial plant disease specialist, Faye Dokken-Bouchard. “The fact that
we were seeing more samples with stripe rust tells us there was more disease out there in general but that didn’t seem to be the disease that was causing the most problems,” she says.
Stripe rust is much more prevalent in Alberta than other parts of Western Canada, and is most severe in the Lethbridge area, where its incidence has increased over the past decade. The predominant winter wheat variety in recent years has been Bellatrix which is very susceptible to stripe rust. Bellatrix has been replaced, for the most part, by the variety Radiant which carries the Yr10 gene for resistance to stripe rust.
A substantial number of the winter wheat fields in southern and central Alberta were infected
with stripe rust by October last year. “We recommend that producers in this area plant resistant varieties,” says Kequan Xi, a cereal pathologist at the Lacombe Research Centre, who adds that fungicides are also effective if there are early signs of infection, but can be something of a gamble. “It’s another line of defence but fungicide application needs to be done in a narrow window of time to be effective. On the other hand, if the disease is not severe or the weather turns dry afterwards, it can curtail the disease spread to a certain extent. In this case you may not be able to maximize the cost effect of fungicide application.”
The search for new stripe rust-resistant varieties continues as scientists try to keep one step ahead of what is proving to be a very adaptable pathogen. “The stripe rust pathogen adapts very rapidly to resistance genes and to environment, as well,” says Denis Gaudet, a research scientist at AAFC’s Lethbridge Research Centre, who is heading up stripe rust research at the facility. “I consider it one of the most serious pathogens in wheat at the present time.”
A major resistance gene in Radiant winter wheat, Yr10, has recently decreased in effectiveness. There is also evidence that the pathogen may be able to overwinter under certain conditions, and there is a suspicion that this contributed to a serious outbreak in Alberta in 2006.
Research at Lacombe has provided evidence that stripe rust inoculum can infect the small amount of green tissue present in fall-seeded winter wheat crops, and as long as the crop has adequate snow cover, it can survive in that green tissue until the following spring. This provides an additional, early source of the pathogen, besides that which blows in from elsewhere, increasing the disease pressure.
Tests also showed that spring wheat seeded on fields adjacent to winter wheat had higher levels of stripe rust, which developed much earlier in the season, than fields where spring wheat was not planted adjacent to winter wheat. “This probably means that there is quite a bit of stripe rust inoculum sitting in the winter wheat field ready to infect the subsequent spring wheat crops,” says Xi.
In view of these factors and increasing incidences of stripe rust, especially in Alberta, researchers from across Western Canada have added stripe rust as a Priority One disease, which gives it more urgent consideration for purposes of breeding priorities and new variety registration. The disease’s priority status will go into effect in 2017 to provide time for breeders to develop additional resistant varieties.
Not to say that there aren’t a lot of resistant varieties already available. All durum wheat varieties seem to be naturally resistant to stripe rust and adult plant resistant genes (which tend to be more stable and durable than race-specific genes like Yr10) have been found to be effective against several different stem and leaf rusts, so have been widely incorporated into spring wheat and winter wheat varieties. “One gene in particular that breeders have been using is the Lr34/Yr18,” says Gaudet. “It provides resistance to leaf rust and molecular studies have shown that this gene also provides resistance to stripe rust. That gene is fairly widespread in many of our current wheat varieties.”
There is also increasing evidence, coming out of the stripe rust research being conducted at Lacombe, that the two pathogens causing wheat and barley stripe rust, respectively, can cross infect wheat and barley species.
Incidence of stripe rust in barley has generally been much lower than in wheat, which has meant there has been less concern about it, but in barley research plots last year stripe rust severity was much higher than usual, says Xi. “We have tried to determine if there is a cross infection of stripe rust disease between wheat and barley.” He cautions against being too complacent about the potential of stripe rust to cause significant yield losses in barley crops.
The research, which was conducted in the greenhouse, showed that both varieties of wheat and barley could be infected with the same pathogen, regardless of whether the genotype was previously associated with the disease in wheat or barley. “This remains to be proven in field conditions, but in the greenhouse we have determined that they will definitely jump the chain between the two,” says Xi.
The stripe rust pathogen adapts quickly; new resistance genes will be vital in the longterm