Xianming Chen, a research plant pathologist with the USDA Agricultural Research Service in Pullman, released his first stripe rust forecast of the season in January, and it’s good news for wheat growers.
However, you might not know that if you just read the first sentence of his summary, which says, “the current forecast is that highly susceptible winter wheat varieties would have about 30 percent yield loss, in the middle of moderate epidemic range (20 to 40 percent yield loss).”
Producers certainly don’t think a 30 percent yield loss sounds like good news. However, Dr. Chen’s forecast models are based on yield loss of a highly susceptible variety, PS 279, which is not grown commercially in the Pacific Northwest. He uses PS 279 because it has been used in Northwest stripe rust research for a long time, which allows him to predict the maximum yield loss under current conditions by comparing average temperatures in November and December 2015 to temperatures and yield loss of PS 279 in previous years.
Some translation from PS 279 to varieties actually grown is required, and the good news in this forecast is that yield loss of susceptible commercial varieties should not exceed 15 percent, depending on their degree of susceptibility and weather conditions through the rest of the spring. Of course, the prediction models assume weather conditions during the rest of the spring will be conducive for rust development, which may or may not occur.
In addition to weather, the availability of stripe rust spores to infect wheat plants is required for an epidemic to get started. Stripe rust levels were low last summer, and dry fall weather caused much of the wheat crop to emerge later than normal. This late emergence was not conducive for stripe rust development and very little fall infection occurred, resulting in very few stripe rust spores out there to begin the epidemic. Because of this, I believe the potential for stripe rust damage this year is very low. If winter hangs on and cool temperatures persist over the next six weeks, the potential for stripe rust will be less than if temperatures warm and spring comes early to eastern Washington. We will continue to monitor the weather and reports of stripe rust as winter turns to spring, so stay-tuned for the next forecast in early March, which as Dr. Chen mentions, is usually more accurate than this early forecast.
Given the current forecast, winter varieties likely won’t benefit from an early fungicide application this year, especially if they are resistant or moderately resistant. However, if you’re planning to plant spring wheat, remember to select the most resistant variety that performs well in your area. Rust ratings on current varieties can be easily found on our Variety Selection Tool.
• Author Tim Murray, Extension Plant Pathologist, can be reached by email at firstname.lastname@example.org, by phone at (509) 335-7515, or through Twitter @WSUWheatDoc.