Crown rust, also known as leaf rust, is caused by a fungus, Puccinia coronata f.sp. avenae. The crown rust fungus is specific to cultivated oat, wild oat and a few other wild grasses, but will not infect wheat, barley or rye.
Crown rust reduces oat yield and causes thin kernels with low test weight – factors that greatly reduce milling quality. Loss due to disease can reach 100 per cent if infection is early, if it is a susceptible cultivar, and if weather conditions are favourable for the development of fungal spores and their spread.
Losses and Symptoms
Losses have been more severe in Manitoba compared to Saskatchewan, for the time being at least. The problem area in Saskatchewan is the southeast. In this region, oat crops are more likely to be exposed to the rust fungus moving in from the south.
Symptoms of crown rust include orange pustules of spores developing on oat leaves (see photos). Each pustule contains thousands of spores that can be spread to neighbouring plants and produce a new pustule in only seven to 10 days under ideal conditions.
The highest risk areas for crown rust are in Manitoba and southeast Saskatchewan. Late planting of oats followed by humid warm weather are the most favourable conditions for disease development. Some oat cultivars bred for resistance to crown rust are no longer effective at warding off infection, due to the fact that the rust population has developed new races that have overcome the resistance. Because of this breakdown in varietal resistance, growers in high risk areas will need to switch to newer resistant varieties and/or incorporate other management practices to reduce disease risk.
The rust pathogen is not seed or soil-borne. The primary means of infection is from rust spores moving up on air currents from the southern part of the United States. The onset and severity of rust infection in the eastern prairies is dependent on what happens on southern crops. The United States Department of Agriculture (USDA) produces a cereal rust bulletin that monitors rust development in the U.S., which is useful in predicting the risk to Canadian crops. Provincial specialists use this tool to keep producers and agronomists updated on the risk of crown rust as the season progresses.
Within the rust fungal population, there are a number of different "races" that developed and are specific to oat varieties carrying certain rust resistant genes.
Developing crown rust resistant varieties is an ongoing battle for cereal breeders. As soon as new varieties are developed with specific genes for resistance, it puts selection pressure on the rust population which develops new races that overcome this resistance. The goal of cereal breeders is to "pyramid" genes for resistance, meaning that several genes are incorporated into one variety to extend the breakdown of resistance. Refer to Varieties of Grain Crops for more information on oat varieties with crown rust resistance.
Other ways can farmers manage crown rust
Other than using resistant varieties, one of the key ways to avoid severe infection is to plant oat crops early. The premise for planting early is that the crop should be advanced enough, by the time rust spores arrive on the eastern prairies, that crops will not suffer significant yield or quality loss.
Foliar fungicides are another tool that growers have to manage. There are a number of fungicides registered on oats for crown rust. Refer to the Guide to Crop Protection for more information on fungicides registered on oats.
The ideal timing for application is to spray at flag leaf emergence to protect the flag leaf. Rust can develop very quickly, so earlier application may be necessary if conditions are conducive to rapid spreading of the rust. Once the flag leaf is covered with spots, it is too late to apply fungicide. The decision whether or not to apply fungicide must take into the account the availability of rust spores moving up from the south, and weather conditions. Spraying fungicide will not be necessary in all years or locations.
Bailey, K.L., Gossen, B.D. ,Gugel, R.K., Morrall, R.A.A. 2003, Diseases of Field Crops in Canada, CPS Press, Saskatoon, SK. pages 75-77.
Chong, J. 2006, Crown Rust in Oat in Western Canada in 2005. Canadian Plant Disease Survey, Volume 86, pages 66-67