By Carolyn King
A crown rust-susceptible oat cultivar (left) and a rust- resistant one (right) grown in an oat trial in Winchester, Ont. Photo courtesy of Weikai Yan, AAFC.
“Crown rust is probably the most important disease of oats in Canada, from Quebec through to Saskatchewan,” says Albert Tenuta, a plant pathologist with the Ontario Ministry of Agriculture, Food and Rural Affairs. Oat crown rust can be especially troublesome in southern Ontario. Fortunately, tools are available to help Ontario growers manage the disease, including the latest rust-resistant varieties released by the federal oat breeding program in Ottawa.
“The disease is pretty easy to identify: it produces orange pustules on both sides of the leaves. Many growers call the pustules ‘little volcanoes,’ ” Tenuta explains. Those little volcanoes release thousands of spores, spreading the disease to other plants and other fields.
He notes, “Ideal conditions for oat crown rust are mild to warm daytime temperatures, so sunny days at about 20 C to 25 C, and moderate nighttime temperatures around 15 C to 20 C, along with good dews and adequate moisture.”
Oat crown rust damages the oat leaves, causing up to 40 per cent yield losses. Tenuta says, “The higher up on the plant where the damage occurs, particularly when the disease starts getting into the flag leaf, the greater the potential yield impact. The disease can also reduce grain quality, causing shrivelled grain. As well, rust infections can impact the plant’s tolerance to other stresses.”
The pathogen that causes oat crown rust is Puccinia coronate f. sp. Avenae. The name crown rust comes from one of its spore types, called a teliospore, which has little prongs on the top of the spore, forming a crown-like structure. The pathogen has a complicated life cycle involving both asexual and sexual reproductive cycles and several different types of spores.
“According to our plant pathologist, Dr. Allen Xue, the crown rust population in Ontario is more diverse than that in Western Canada because we have two sources of the disease in Ontario,” explains Weikai Yan, oat breeder with Agriculture and Agri-Food Canada (AAFC) in Ottawa.
One source of the disease is wind-blown spores that come from the southern United States, bringing the disease to oat crops in both Western and Eastern Canada in the spring. This spore type is reproduced asexually – the spores are clones. So one generation is usually genetically the same as the next, although mutations can occasionally occur so new races sometimes develop.
Ontario also has a local source of spores: rust infections on a shrub or small tree called common buckthorn or European buckthorn (Rhamnus cathartica), which is widespread in southern Ontario. Tenuta explains, “Like other rust pathogens, crown rust infects not only a host crop, which is oat in this case, but it also has part of its life cycle on an alternate host, buckthorn. The alternate host plays an important role in terms of sexual reproduction, and that’s where a lot of the variation in crown rust races comes from. So areas with more buckthorn are often where we’ll see more races and the greatest crown rust risk for producers.”
Managing oat crown rust
One of the main ways to control the disease is to use rust-resistant oat varieties. “There are many different resistant genes available, and their effectiveness in the province and in North America changes, especially because buckthorn allows for genetic recombination, allowing new crown rust races to develop quickly,” Tenuta says.
“What we typically see [as the pathogen evolves], is that an oat variety with a very effective resistance gene will gradually show more crown rust over time. That is an indication that new pathogen races are developing that can overcome the resistance gene.”
He suggests trying two or three different oat varieties on your farm to get an idea of the effectiveness of their resistance in your fields. “There can be regional differences [in an oat variety’s performance] due to differences in things like buckthorn numbers, pathogen races and weather conditions.” The website gocereals.ca provides information on oat variety performance, including crown rust resistance, in the different regions of Ontario.
Another practice to help manage the disease is to remove buckthorn plants near oat fields. That will decrease the risk of crop infection and the risk of development of new races.
If your oat variety doesn’t have good crown rust resistance, then applying a foliar fungicide is important, especially to protect the flag leaf. “We have very good, effective fungicides available for crown rust,” Tenuta says. “On an annual basis, the very worst scenarios I see with crown rust are where the resistance gene is no longer effective and where a fungicide was either not applied or the application was too late to give good control.”
He also advises diligent scouting. “Oat producers need to be out in their fields every year evaluating the performance of their management tools for crown rust – their varieties and their fungicides. If you see things changing, that is an early warning that maybe the effectiveness of the resistance gene in the variety is no longer going to be adequate for your fields.”
Ongoing breeding for rust resistance
“The crown rust pathogen has virulence genes, which are like swords, and the plants have resistance genes, which are like shields. Different races of the pathogen have different virulence genes. So you need to have the right type of shields to protect the plants from the attack of the races in your area,” Yan explains. Oat breeders have to continually develop new rust-resistant varieties that are able to fight off the latest crown rust races.
Yan’s oat breeding program is responsible for breeding cultivars for Eastern Canada. “In Eastern Canada, the crown rust pressure is mainly in southern Ontario, anywhere south of the Ottawa Valley. There is some rust pressure in the Montreal area as well, but generally rust is not a problem in northern Ontario, most regions of Quebec and the Maritimes,” he says.
“So, for our oat breeding program, we divide Eastern Canada into the rust region and the non-rust region. For southern Ontario, crown rust is always the single most important issue. Every year, the first trait we select for is crown rust resistance. If a line doesn’t have crown rust resistance, we do not keep it.”
Yan screens his breeding lines in field plots at AAFC’s Ottawa Research and Development Centre, so he selects lines with resistance to the current crown rust races in the region. He also collaborates with Xue, who does growth chamber tests to determine which specific resistance genes are present in each of Yan’s promising lines. As well, the promising lines are tested at the University of Saskatchewan in Saskatoon and at AAFC’s Morden Research and Development Centre in Manitoba to make sure the lines have a wide spectrum of resistance to the pathogen, including western Canadian crown rust races.
Xue also conducts an annual survey in farmers’ fields across Canada’s oat crown rust regions and tests the samples for virulence against a wide range of resistance genes. As a result, Yan and other oat breeders have up-to-date information on changes in Canada’s crown rust races.
Yan’s breeding program is putting as many different crown rust resistance genes as possible into each new variety for southern Ontario. That gives the varieties more durable resistance and the ability to protect themselves from many different virulence genes.
“Since 2009, we have released 11 varieties that are resistant to crown rust. So far, we have not found any rust on some of these varieties, for example, Almonte and Kolosse. These varieties are likely to carry resistance genes Pc61, Pc96, and Pc91. Pc94 has also been incorporated in some of our newer varieties to be released in the next few years,” Yan says.
Yan says the program’s 11 rust-resistant varieties include: “Optimum released in 2009, Bullet released in 2010, Roskens in 2011, Almonte in 2013, Oaklin in 2013, Richmond in 2013, Pontiac in 2014, Nicolas in 2014, Kolosse in 2015, Noranda in 2015 and Blake in 2015.”
“Most of these varieties were selected from crosses made by the oat breeder who was here at Ottawa before me: Dr. Art McElroy. My contribution is to select from these populations the rust-resistant var-ieties that are high yielding and good quality, and to release them.”
Yan adds, “Those 11 varieties are pretty high yielding. For instance, Bullet is now the most popular oat variety in southern Ontario; its yield is 16 to 20 per cent higher than the means of the yield trials. Nicolas has a 20 per cent higher yield in Quebec than the control cultivars. Bullet is already having a very good impact on oat production and we think Nicolas will have a really good impact in Quebec and northern Ontario. So we’re pretty happy with this.”