Stripe rust update 05/21: Disease has been developing in Washington

From Dr. Xianming Chen

Since the last stripe rust update on May 7, stripe rust has been developing in the Pacific Northwest (PNW), especially in Washington. After the update, we received a report of stripe rust in commercial fields of winter wheat in Franklin County.

On May 9, we checked the fields planted with Piranha CL+ and Nova AX, and stripe rust was easy to find although the incidence was less than 1%. The Nova AX field was sprayed with fungicides a day before, and the Piranha field was sprayed on that day. We checked more than ten additional commercial fields of winter wheat fields in Franklin and Adams counties and found stripe rust in three fields with incidence less than 0.1%

On May 15, we were checking our experimental fields around Pullman. Stripe rust was easier to find than a week earlier on spreader rows planted with a highly susceptible winter wheat variety, but still below 1% incidence. No rust was found in commercial winter wheat fields in the Palouse region.

On May 20, we were taking stripe rust data in our winter germplasm screening nurseries at the Central Ferry station. Winter wheat reached the flowering stage (Feekes 10.51-10.53) and barely reached Feekes 10.53-10.54. As expected, stripe rust reached 90% to 100% severity on susceptible varieties of both wheat and barley. The stripe rust fungus turned into the black telial stage, which no longer produces infectious, orange-colored spores. Some strains of the stripe rust fungus turn into the telial stage faster than others.

The weather conditions of cool temperatures and frequent showers in the past two weeks were extremely conducive to stripe rust. The low temperatures did not allow high-temperature adult-plant (HTAP) resistance to be fully effective. So, the varieties with moderate levels of HTAP resistance without any effective all-stage resistance, like Piranha CL+ and Nova AX, have been susceptible. For HTAP resistance to play a significant role in reducing stripe rust, temperatures need to be above 50 F at night and above 70 F in the daytimes. The weather conditions forecast for the next two weeks will be continually favorable to the disease. Therefore, our recommendations are the same as issued in the last two

updates (April 18 and May 7). Fungicide applications are recommended for winter wheat fields planted with susceptible and moderately susceptible varieties (stripe rust ratings 6 to 9 in the seed-buying guide) before flowering. If fields sprayed with fungicides more than three weeks ago, check the fields for active rust like in Figure 1. For spring wheat, consider fungicide application at the time of herbicide application for the fields planted with susceptible (stripe rust ratings 8 and 9) or moderately susceptible (stripe rust ratings 6 and 7) varieties.

Stripe rust in the country

Since the last update (May 7), stripe rust has not been reported in any additional states. Thus, only eight states, Louisiana, Texas, Georgia, Washington, Oregon, California, Oklahoma, and Kansas, have been reported to have stripe rust so far. Even in these states, stripe rust has appeared at relatively low levels compared to last year, except California. However, stripe rust may still have time and conditions to develop to epidemic levels in some regions.