The risk from stripe rust increased nearly 10% in Dr. Xianming Chen’s, USDA-ARS plant pathologist, most recent forecast released March 1. His current forecast calls for 51% yield loss on highly susceptible varieties compared with 42% in the January forecast. These predictions are based on weather data from November 2023 through February 2024. In my last update, I noted that the forecast did consider the amount of stripe rust present in commercial fields; unfortunately, this forecast includes survey data that identified multiple fields near Walla Walla, Almira, and southern Adams County along highway 21 with actively sporulating stripe rust pustules. These observations demonstrate that the stripe rust fungus survived the winter and spores are available to spread the rust. Such early appearance of stripe rust in commercial fields has not been observed since February 2011.
It’s important to remember that these yield loss estimates are based on a highly susceptible variety that is not grown commercially. However, that does not reduce the potential severity of stripe rust this season, which for current commercially grown varieties rated 5-9 (moderately to highly susceptible) ranges from 9 to 33% yield loss without fungicide application. Consequently, early season prophylactic fungicide application is recommended for fields planted with winter wheat varieties rated 5 to 9 (moderately to highly susceptible) at the time of herbicide application. Varieties rated 1-4 likely won’t benefit from a fungicide application based on Dr. Chen’s research.
A second fungicide application may be needed 20 to 30 days after the first application, depending on the material applied and the weather. Whether a second application is needed is best determined by scouting fields and looking for active stripe rust on 1-5% of the flag leaves. In general, single mode of action fungicides will provide about 3 weeks of protection, and those with two or three modes of action will provide another 7-10 days of protection.
Given this current prediction, I encourage you to carefully consider spring wheat stripe rust resistance variety ratings (available on the smallgrains.wsu.edu website) and select the most resistant variety possible, preferably rated 1-4, for your production area. These varieties are unlikely to benefit from a fungicide application.
Stay tuned for more rust updates as conditions change. In the meantime, you can find additional information about stripe rust, including photos showing rust percentage under the Foliar Fungal Diseases and updated rust ratings for winter and spring wheat varieties in my presentation “Diagnosing and Managing Diseases of Wheat” from the 2023 Wheat Academy in the Disease Resources section of the WSU Wheat and Small Grains website.
For questions or comments, contact Tim Murray via email at tim.murray@wsu.edu, via phone at (509) 335-7515, or by following him on Twitter @WSUWheatDoc.