May 26 stripe rust update

Stripe rust found throughout Pacific Northwest wheat fields

By Xianming Chen
Plant Pathologist, USDA-ARS

We were checking wheat and barley fields on May 25 in Latah County of Idaho and Whitman, Spokane, Lincoln, Grant, Douglas and Adams counties of Washington. Winter wheat ranged from Feekes 10 (boot) to Feekes 10.5.4 (milk), and spring wheat and barley ranged From Feekes 1 to Feekes 8. With adequate moisture, crops looked good. In our experimental fields around Pullman, Wash., stripe rust developed to more than 90 percent severity on susceptible varieties of winter wheat and appeared on spring wheat with less than 1 percent severity. Similarly in our nurseries at Lind (Adams County), Wash., stripe rust was up to 80 percent on susceptible winter wheat varieties and appeared on spring wheat. In most commercial fields, stripe rust was generally at low severity levels (less than 5 percent) in winter wheat fields thanks to resistant varieties and early applications of fungicides. A field in western Spokane County and a field in Lincoln County along Highway 2 had significant rust, with susceptible reactions and about 20 percent severity. Another field between Wilber and Odessa, Wash., along Highway 21 in Adams County, had severity of about 60 percent. Stripe rust was found in many checked fields on low leaves and at low incidence (less than 1 percent).

With the recent cool, wet weather and forecasted weather conditions for the next few weeks favorable to stripe rust and the redevelopment of the disease in some winter wheat fields, a second application is necessary for fields grown with moderately susceptible to highly susceptible varieties (ratings 6-9 in the Seed Buyer’s Guide). If a field was sprayed three to four weeks ago, please check fields. If active stripe rust pustules are found, a second fungicide application is definitely needed. Under the currently heavy rust inoculum and low temperature conditions, fungicide application on moderately resistant varieties (rated 4-5) can be beneficial to reduce some yield loss. The timing of a second application is a complicated issue as it’s related to the current rust situation in the field; variety resistance level; growth stage; how long ago the field was sprayed with fungicide and which fungicide was used; weather conditions (rain and wind); and application method (ground vs. air). The general guidelines are: If susceptible and active rust pustules are easily found, apply a fungicide as soon as possible. Under such conditions, if air application is not possible within a week, use ground application. For spring wheat, fungicide application at the time of herbicide application is needed for fields of susceptible and moderately susceptible varieties. As barley stripe rust is low, fungicide application is generally not recommended unless stripe rust is seen in the field.

Stripe rust in the U.S. and Canada

Stripe rust has been reported in 29 states: Texas, Oregon, Louisiana, Arkansas, Oklahoma, Washington, Colorado, Kansas, Mississippi, Georgia, California, Virginia, Montana, Indiana, Idaho, North Carolina, Tennessee, South Dakota, Kentucky, Illinois, Nebraska, Minnesota, Delaware, Florida, Michigan, Ohio, Wisconsin, Utah and North Dakota. Stripe rust has also reported in Alberta and Ontario provinces of Canada. The distribution of stripe rust in the U.S. and Canada is almost like the situation in 2010 where the disease was extremely widespread.