Stripe rust is actively developing in Washington, with recent observations in Franklin (commercial field), Adams (commercial field), Whitman (experimental field), and Garfield (experimental field) counties. While initial incidences in commercial winter wheat fields were low (less than 1% to 0.1%), experimental plots with susceptible varieties have seen severity reach 90-100%. The fungus is now entering its non-infectious black telial stage in some areas.
Cool temperatures and frequent showers over the past two weeks have created ideal conditions for stripe rust, limiting the effectiveness of high-temperature adult-plant (HTAP) resistance in some wheat varieties. The forecast indicates continued favorable conditions for the disease.
Recommendations for farmers remain consistent:
- Winter wheat: Apply fungicides to susceptible and moderately susceptible varieties (ratings 6-9) before flowering. Recheck fields sprayed over three weeks ago for active rust.
- Spring wheat: Consider fungicide application at the time of herbicide application for susceptible (ratings 8-9) or moderately susceptible (ratings 6-7) varieties.
Stripe rust ratings can be found on the Variety Selection Tool located on the WSU Wheat and Small Grains website or by visiting the Seed Description & Resources page of the Washington State Crop Improvement Association’s website.
Nationwide, stripe rust has only been reported in eight states (Louisiana, Texas, Georgia, Washington, Oregon, California, Oklahoma, and Kansas) and generally at lower levels compared to last year, except in California. However, there is still potential for epidemic development in some regions given suitable conditions.
More information about stripe rust, including photos showing rust percentages and a table listing fungicides with their active ingredients, use rates, and growth stage restrictions, is available under the Foliar Fungal Diseases in the Disease Resources section of the WSU Wheat and Small Grains website. Additional information is also available on Dr. Chen’s website.
HTAP Resistance
High-temperature adult-plant resistance in wheat, crucial for combating stripe rust in the Pacific Northwest, does not activate at a single temperature. Instead, it gradually increases like a dimmer switch, becoming more effective as temperatures rise. Generally, for HTAP resistance to become active when wheat plants are nearing jointing, nighttime temperatures should be in the 50s and daytime temperatures in the 70s. Both day and night temperatures are critical, and cooling periods can diminish the resistance. However, these temperatures also favor stripe rust development, which thrives between 46°F and 75°F.
The specific temperature threshold for HTAP activation varies significantly depending on the wheat variety and its genetic makeup. With over 30 HTAP resistance genes in PNW wheat, some varieties have only a few, while others have many. These genes differ in their sensitivity to growth stage and temperature. Some, like those in Madsen wheat (SWWW), are effective earlier and less affected by temperature fluctuations. HTAP resistance is distinct from all-stage resistance, which is present throughout the plant’s life and influenced by stripe rust races. Modern wheat varieties often combine both types of resistance to provide robust protection. Varieties like Seahawk (SWSW) and Expresso (HRSW) feature combinations of all-stage and HTAP resistance genes, offering effective defense throughout the plant’s growth. Ongoing breeding efforts aim to identify optimal gene combinations that ensure consistent resistance against evolving stripe rust strains.
Additional Resources
USDA’s Dr. Xianming Chen’s May 21, 2025 stripe rust update
Early May 2025 stripe rust update summary
March 2025 stripe rust update summary
Co-authored with Tim Paulitz, USDA