By Xianming Chen
First forecast for 2016
The current forecast is that highly susceptible winter wheat varieties would have about 30% yield loss, in the middle of moderate epidemic range (20-40% yield loss). Based on this forecast, currently grown varieties would have 0 to 15% yield loss, depending upon the level of susceptibility; and early fungicide application at the time of herbicide application time for winter wheat will generally unnecessary. This forecast is based on the November and December temperatures, and is only for the wheat areas in eastern Pacific Northwest. In early March, we will make another forecast based on the weather conditions of the entire winter season, which is generally more accurate than the early forecast.
On November 10, 2015, we were checking winter wheat fields in Whitman, Adams, Lincoln, Grant, and Benton counties in Washington. No rust was observed. However, we received stripe rust samples from volunteer wheat and grasses from southern Idaho in later October and early November of the last year.
Fungicide tests in 2015
In the winter wheat field near Pullman, WA for fungicide testing, stripe rust was first observed on April 28, about two weeks earlier than normal for this area. The first application of fungicide was conducted at jointing stage on May 6 when stripe rust was less than 1% in some plots, and the second application of some treatments or only one application treatments were conducted at boot stage on May 20, 2015 when stripe rust was less than 5%. As shown in Table 1, stripe rust developed to 80% severity at flowering stage and 100% severity at milk stage in untreated plots. All fungicide treatments significant reduced stripe rust severity (as measured by relative area under the disease progress curve (AUDPC), but had different effectiveness. All fungicide treatments significantly increased test weight and yield except two treatments. The non-treated plots had an average yield of 74.67 bu/A, while the average yields ranged from 80.07 to 100.29 bu/A, increasing by 7.23 to 34.31%.
A similar study was conducted for spring wheat, but no differences in stripe rust severity, test weight, and yield were found between any of the treated and non-treated plots (data not shown), because of low rust and low yields due to the long dry and hot summer.
Stripe rust yield losses of wheat varieties in 2015
Stripe rust and yield differences in non-sprayed and fungicide sprayed plots of winter wheat varieties are shown in Table 2.
Of the 24 winter wheat varieties tested including susceptible check ‘PS 279’, 5 varieties (PS 279, Xerpha, Tubbs 06, Mary, and ORCF-103) had significant differences in stripe rust severity, none of the varieties had significant differences in test weight, and only one variety (PS 279) had significant differences in yield. Stripe rust caused 28% yield loss on the susceptible check (PS 279) and up to 10% yield losses on commercially grown varieties. Under such level of stripe rust epidemic, only the susceptible check was rated great than 1, for which fungicide application was needed. All commercial varieties were rated 1, and no fungicide application was needed.
The data can be used to select stripe rust resistant varieties to plant and to determine if fungicide application is needed for a variety based on the relative yield loss and potential epidemic level. The current forecasted epidemic level (30% yield loss on susceptible varieties) for 2016 is about the yield loss level for susceptible
varieties occurred in 2015. Therefore, fungicide application will be generally unnecessary for winter wheat varieties.
Please note that the field was only sprayed once in early May, and stripe rust was not completely controlled by the one-time early application of fungicide. The yield loss could be under estimated for the susceptible check.
In the spring wheat variety nursery, stripe rust did not develop to a uniform and adequate level, and no significant differences in stripe rust severity, test weight, and yield were observed (data not shown).