Watch For Crown Rust On Oats in the Southeast

Source: Saskatchewan Agriculture and Food

Researchers are warning oat producers to watch for crown rust this season. The level of crown rust has been increasing in recent years in Manitoba and south-eastern Saskatchewan, says Saskatchewan Agriculture and Food Plant Disease Specialist Penny Pearse.

“The crown rust outbreak in 2005 was one of the most severe we've witnessed in many years," she says. "The 2005 season was characterized by late planting of oat, and humid and warm weather conditions that were favourable for crown rust infection. Furthermore, oat cultivars bred for resistance to crown rust are no longer effective at warding off infection, because the rust population has developed new races that have overcome the resistance. Because current oat cultivars are no longer effective at resisting crown rust, growers will need to incorporate other management practices to reduce disease risk.”

Crown rust—also known as leaf rust—is caused by the fungus Puccinia coronata f.sp. avenae. The crown rust fungus is specific to cultivated oat, wild oat and a few other wild grasses, and will not infect wheat, barley or rye. “Crown rust reduces oat yield, and causes thin kernels with low test weight. This greatly reduces milling quality. Losses due to the disease can approach 100 per cent if infection is early, if it is a susceptible cultivar, and if weather conditions are favourable for the development and spread of fungal spores,” says Pearse. So far, losses have been most severe in Manitoba. The problem area in Saskatchewan will be the southeast, where oat is a preferred crop and is more likely to be exposed to the rust fungus moving in from the south.

Symptoms of crown rust include orange pustules developing on oat leaves. Each pustule contains thousands of spores that can spread to neighbouring plants and produce new pustules in only seven to 10 days under ideal conditions.

This raises the question: why are our current oat cultivars no longer resistant? “Within the rust fungal population," Pearse explains, "there are a number of different races that have evolved to overcome the rust resistance genes in our current oat varieties. Almost all oat cultivars currently grown on the eastern prairies rely on a single gene for resistance: Pc68. The outbreak of oat rust in 2005 proves this gene is no longer effective.

“Dr. James Chong, a pathologist with the Cereal Research Centre in Winnipeg, has determined the frequency of the various rust races collected from infected plants. In 2005, 73 per cent of isolates from cultivated oat were virulent against the Pc68 gene, compared to 39 per cent of isolates from 2004, and only 12 per cent in 2003. This is evidence that the rust race that can overcome the Pc68 gene has increased rapidly in the rust population in only a few years.”

The primary means of infection is rust spores floating up from the southern United States on air currents. The onset and severity of any rust infection in the eastern prairies is dependent on what happens to southern crops.

“If there is a high proportion of the race virulent against the Pc68 gene in the rust population developing in the U.S., oat crops on the eastern prairies are likely to suffer," she explains. "The United States Department of Agriculture (USDA) produces a cereal rust bulletin that monitors rust development in the U.S., and we can use it to predict the risk to Canadian crops. To date, rust levels have been reported as low in the southern states; however, this is not a guarantee that some spores won’t find their way to Canada later this season. Provincial specialists will continue to keep producers and agronomists updated on the risk in 2006 as the season progresses.”

Developing crown rust resistant cultivars is an ongoing battle for cereal breeders. As soon as new cultivars are developed with specific genes for resistance, the rust population begins to develop new races to overcome this resistance. The goal of cereal breeders is to “pyramid” genes for resistance, meaning that several genes are incorporated into one cultivar to extend the breakdown of resistance or to find less specific, moderate genes for resistance that will delay the breakdown.

Two new cultivars, Leggett from Agriculture and Agri-Food Canada in Winnipeg, and Hi-Fi from North Dakota State University , have been registered in Canada , but certified seed is not yet available, explains Dr. Brian Rossnagel, an oat breeder at the Crop Development Centre at the University of Saskatchewan . New sources of resistance have been identified in wild oats collected in Europe , the Middle East and North Africa, and are being incorporated into the breeding programs. Several advanced lines with resistance that have been developed at the Crop Development Centre at the University of Saskatchewan and Agriculture and Agri-Food Canada are in co-op tests in 2006.

One of the key ways to avoid severe infection is to plant oat crops early.

“By planting early, the crop should be advanced enough by the time the rust spores arrive in the eastern prairies that it will not suffer significant yield or quality loss," Pearse explains. "A second option is for growers to be more selective in the cultivars they choose to plant. Although most of the cultivars depend solely on gene Pc68 and won’t be effective, a few other cultivars have other genes. Until Leggett and Hi-Fi are available, the best choices would be slow-rusting cultivars such as CDC Boyer or CDC Dancer.

“Another tool that growers have to manage rust is foliar fungicides. There are a number of fungicides available that contain the active ingredients propiconazole and/or trifloxystrobin. Ideally, you should spray at flag leaf emergence to protect the flag leaf. Rust can develop very quickly, so once the flag leaf is covered with spots, it is too late to apply fungicide.”

Oat producers need to be aware that crown rust may be a problem in oats in 2006. Management practices as well as crop scouting should be implemented to reduce risk.

For more information, contact:

Penny Pearse
Provincial Plant Disease Specialist
Saskatchewan Agriculture and Food
(306) 787-4671

or

Dr. Brian Rossnagel
Oat Breeder, Crop Development Centre
University of Saskatchewan
(306) 966-4976