Blog: The Wheat Sheaf
Stripe Rust in Canada
Reem Aboukhaddour, Research Scientist - Cereal Pathology, Agriculture and Agri-Food Canada talks stripe rust in wheat.
Stripe Rust in Canada
Stripe rust in Canada is influenced by epidemics in the United States. The pathogen’s spores for Alberta are mainly airborne from the Pacific Northwest (PNW) regions of the United States (U.S.), such as Washington, Idaho and Oregon. In recent years, new virulent races of stripe rust were reported to cause damage on wheat in the Great Plains of USA. These new races travelled from the Northern Great Plains to reach parts of Canada including Saskatchewan, Manitoba and Ontario. Based on wind directions, geographical proximity and cropping system, early and severe infection in southern states should alert for the disease in Canada.
Southern Alberta is a hot spot for stripe rust because of its close proximity to the PNW regions of the US. PNW allow year round survival of stripe rust and therefore provide the main source for pathogen inoculum reaching Alberta. If significant outbreaks to occur in the PNW in April or May, then stripe rust spores can reach southern Alberta within few days. The spores can travel a high altitude with western wind for long distances and are then showered down with rain on wheat crops in southern Alberta causing infection, something we usually call “spores showers.” Spores can eventually travel with wind trajectory to central Alberta and other parts of Western Canada.
Stripe rust infection can be first seen in southern Alberta in early to mid-June and in central Alberta by early to mid-July, but if the pathogen is overwintering then earlier infection can be seen. Stripe rust can be more serious if it overwinters in Alberta, as infection can be established early on in the growing season. Early morning dew on the leaves coupled with cool temperature is sufficient for stripe rust to cause infection regardless of the daytime temperature.
Stripe Rust in Spring and Winter Wheat
All fungal pathogens have the capacity to change in virulence. In particular, rust pathogens are very specialized and specific in nature, meaning that the plants have specific genes that can upon recognizing the pathogen trigger, have a defense response and inhibit disease development. However, if the pathogen is confronted by the same cultivar (genes) for prolonged time it can mutate to avoid wheat resistance and therefore cause disease and break down the resistance in that wheat cultivar. The cultivar itself is not changed; the pathogen is changing to defeat resistance.
Since 2000, stripe rust has changed and new races that have adapted to higher temperatures were reported, causing a lot of damage all around the world. Luckily, there are few identified resistance genes that are durable and still effective in our Canadian wheat, but relying completely on limited resistant sources will eventually render them ineffective.
From a genetic stand point, both winter and spring wheat can be susceptible. We have resistant varieties in both of our winter and spring wheat. If stripe rust overwinters on winter wheat, then winter wheat will be exposed to early infection in the spring and with conditions that favor disease development and the disease will build up on it causing damage. Protecting winter wheat will minimize the damage on spring wheat too by reducing the load of spores that can move from winter to spring wheat.
The only effective way to minimize stripe rust infection on winter or spring wheat is by having effective resistant varieties. Some experiments to evaluate fall fungicides application in protecting winter wheat from early infection is being done, but it still too early to make conclusions on the results.
Breeding toward having multiple seedling resistance genes into winter wheat varieties can protect against the overwintering on winter wheat and therefore protect the crop in the early spring. The seedling resistance however is not necessarily to protect the crop at later development stages, and for that it is important to have the adult plant resistance too. Rust needs the living plant tissues to survive; removing volunteer cereal will reduce stripe rust potential to survive in the fall and winter.
Stripe Rust Management
Producers need to recognize the symptoms of infection with stripe rust first and they need to scout their fields regularly and as early as possible because of the overwintering possibilities. Stripe rust symptoms appear as orange color pustules on the leaves that form stripes later on. Usually if you walk in heavily infected area your shoes or cloths will have that orange color on it.
If the variety grown does not have a good level of resistance to stripe rust and producers spotted the symptoms scattered throughout the field even if the crop is at early stage, then consider fungicide application to slow infection down and maybe another application to protect the upper canopy and the flag leaf. It is crucial to protect the flag and penultimate leaves as together contribute to more than 60 per cent of the yield. Fungicides application will protect the uninfected tissue and will not cure the infected parts; it will not protect the un-emerged part of the leaves too.
Stripe rust is a disease that can be explosive in nature, the long stripes formed on the leaves destroy the protective waxing layer of the crop causing the plant to losses a lot of water and as consequence the leaves dry fast. It does not take much of physical damage to destroy the leaves when it comes to rust. Therefore, it is very important to scout the fields early on and regularly throughout the growing season. Fungicide application to a healthy crop is unnecessary and economically unwise. There is no point of applying fungicides after the grain filling stage as the yield is almost fixed at that stage.
Although stripe rust is not seed transmitted disease, but there is a suggestion that seed treatments may protect the crop at seedling stage from early infection and research is needed to validate this suggestion. Crop rotation is not considered an effective practice in controlling stripe rust as the spores are airborne. Yet, avoiding planting spring wheat beside heavily infected winter wheat can give some protection to spring wheat at the early stage of the plant development. Rotation out of cereal crop and removing volunteer plants can help break down the green bridge effect.