By Saskatchewan Agriculture
By Faye Dokken-Bouchard, PAg, Provincial Specialist, Plant Disease
June 2015 - Producers in Saskatchewan should be on alert for wheat rusts this growing season. This means actively scouting winter wheat and susceptible spring wheat varieties for rust symptoms.
Stripe rust can be identified by the elongated yellow-orange pustules that often extend the entire length of the leaf blade and parallel to the leaf veins. In past years, it has been reported primarily in the southwest and south-central areas of Saskatchewan, whereas stem and leaf rusts of wheat tend to occur in the southeastern regions of the province. This year, stripe rust was reported to be prevalent and widespread in the United States, especially in Kansas and Nebraska, whereas leaf rusts initially were lower but ramped up in Texas and Oklahoma as their growing season progressed.
Rusts require a living host and tend to spread from winter wheat to spring wheat and back to winter wheat across the "green bridge" (both crops must be green for the disease to spread). Harsh winters often (but not always) stop the disease cycle here in Saskatchewan, but the pathogen can overwinter in the southern United States and northern Mexico, Pacific Northwest (PNW) and California, and then blow back into our crops through northward movement of wind currents. Rust spores typically arrive in mid to late June from this route.
According to Kelly Turkington's "Cereal Rust / Wind Trajectory Update," the spread of cereal rusts into Western Canada from locations in the U.S. will depend on the following factors:
- Disease severity and pathotypes at the point of origin;
- Release and turbulent transfer of spores into upper atmosphere air parcels;
- Movement and direction of air parcels;
- Spore survival during long distance transport;
- Deposition of spores over at risk locations in Western Canada;
- Crop growth stage for at risk locations; and
- Prevailing weather conditions for at risk locations.
Throughout May, the risk of stripe rust from the PNW was low, and the risk of stripe and leaf rust from the central Midwest regions of the United States was low to moderate. While the risks are specific to certain locations, scouting is not a bad idea for all cereal producers, even for varieties rated moderately resistant or good, as pathogen populations and resistance may change over time.
Radiant winter wheat is no longer resistant to stripe rust, but Moats and Emerson are still moderately resistant. However, the gene that provides resistance in Moats and Emerson is reported to be more effective at warmer temperatures.
A fungicide can provide control of rust at the appropriate growth stage if the disease is present at economic thresholds. One plant per square metre with stripe rust symptoms may be enough to generate an economic return from fungicide application.