Dr. Xianming Chen, ARS Plant Pathologist, released a brief stripe rust update on May 5, 2020 where he noted the first observation of stripe rust on susceptible check varieties in Whitman County research plots. Rust incidence was less than 1% and present on the lower leaves only, which indicates that infection occurred last fall and the rust overwintered in these plants; again, these were the susceptible check varieties – not commercial varieties grown locally. Dr. Chen commented that this is a pretty normal time for stripe rust to appear, although 1 week earlier than 2019.
In his stripe rust update from 2 weeks ago, stripe rust had only been found on on a susceptible check variety in a rust monitoring nursery near Walla Walla and one commercial field in Lincoln County; thus it appears that recent warm weather has allowed the rust to become active. The combination of active rust lesions (yellow-orange sporulation on leaves) along with cooler temperatures and precipitation we received this week will be favorable for continued rust development.
It may be surprising that we aren’t seeing more rust given the very mild winter weather we experienced and Dr. Chen’s early season predictions were for stripe rust to be severe on highly susceptible varieties with no fungicide application. However, it’s important to remember that these forecasts are based on a highly susceptible check variety with no fungicide application. Fungicide application at herbicide timing on susceptible varieties (those rated 5 and above), and growing resistant varieties (those rated 4 or less) likely has reduced the early season build-up of rust.
There is still plenty of time for stripe rust to rear its head and become a problem, especially in susceptible varieties, i.e. those with ratings of 5 to 9 in the Washington State Crop Improvement Seed Buyer’s Guide and the Variety Selection tool on the Wheat & Small Grains website. As always, we encourage scouting fields of varieties with ratings 1-4 and apply fungicide when 1-5% of plants have active rust; it’s unlikely that varieties rated 1-2 will require fungicide application, but rust may develop on those rated 3-4.
Stay tuned for more rust updates as conditions change. In the meantime, you can find additional information on stripe rust, including photos showing rust percentage under the Foliar Fungal Diseases 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 or via phone at (509) 335-7515 or on Twitter @WSUWheatDoc