Where does stripe rust come from?
Stripe rust, also referred to as yellow rust, is a fungus that can reduce wheat kernel quality and yield and is characterized by raised yellow stripes of spores running along the veins of the leaves (Fig. 1). Stripe rust spores are around 0.02mm in diameter, making them even smaller than the width of a strand of human hair! Because of this, they are easily carried by wind currents originating from the pacific northwest, south-central, and mid-west regions of the United States, and are deposited on successive wheat crops from Texas to Nebraska before finally moving into western Canada. Wind currents from Texas to Nebraska typically travel to the central to eastern prairie region, although occasionally they can travel as far as the northern-most part of the Peace region. Despite the great distance wind-borne spores travel, stripe rust can take less than a day to make its way north, leaving southern Alberta particularly at risk for infection. Occasionally, stripe rust spores overwinter in Alberta winter wheat crops during mild winters, as they can only withstand temperatures from approximately -10 to -17°C. Stripe rust is becoming more of an issue due to virulence shifts in some races of the fungus that are able to withstand higher summer temperatures, as well as cooler winters.
While wheat can be infected at any stage of growth, you can potentially avoid or reduce stripe rust infection by choosing resistant varieties. If that strategy doesn’t work though, it’s helpful to understand how infection occurs. The main factor that invites infection when spores are present are environmental conditions. The fungus thrives in relatively cool summer temperatures, from 7 to 17°C, and requires wet or humid conditions to germinate and infect a new plant. However, there are biotypes of stripe rust that can develop at warmer temperatures.
Once stripe rust infects a plant, the outside environmental conditions have less of an effect on its growth. If the temperature reaches higher than 30°C or excessively dry conditions prevail, rust development slows. Generally, the more moisture, the higher incidence of infection. This includes heavy dew, rain, and/or irrigation. Infection is also exacerbated by cool, moist nights. Nighttime temperature is much more important when determining rate of infection because it is reaching a temperature at which the spores can germinate and infect, while the dry day time environment serves as a resting period when infection doesn’t readily occur. If you’re experiencing infection, there are fungicide application options to consider.
The best thing you can do is stay informed about the projected risk level and the infection levels in the source locations. Being updated on wind trajectories and understanding what conditions are conducive for stripe rust growth and infection will help you stay ahead of infection, allowing you to react more timely if an issue does arise. Resources such as the Wind Trajectory Reports and the Cereal Rust Risk Reports provided by the Prairie Pest Monitoring Network Blog are good tools to use when dealing with stripe rust, but also leaf and stem rust.