Stripe rust in the Pacific Northwest
We were checking wheat and barley fields yesterday 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 good moistures, crops looked good. In our experimental fields around Pullman, stripe rust developed to more than 90% severity on susceptible varieties of winter wheat, and appeared on spring wheat with less than 1% severity. Similarly in our nurseries at Lind (Adams County), stripe rust was up to 80% on susceptible winter wheat varieties and appeared on spring wheat. In commercial fields, stripe rust was generally at low severity levels (less than 5%) in winter wheat fields, thanks to the resistant varieties and early application of fungicides, except few fields. A field in western Spokane County and a field in Lincoln County along Highway 2 had significant rust, with susceptible reactions and about 20% severity. Another field between Wilber and Odessa along Highway 21 in Adams County had severity about 60%. Stripe rust was found in many fields checked on low leaves and at low incidence (less than 1%).
With the cool and wet weather conditions extremely favorable to stripe rust in the past two weeks and forecasted weather conditions for the next two weeks still favorable to the disease, and stripe rust was seen to re-develop in some fields of winter wheat, 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 3-4 weeks ago, please check fields. If active stripe rust pustules (Fig. 1) 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. Timing of second application is a complicated issue as this is related to current rust situation in the field, variety resistance level, growth stage, how long ago the field was sprayed with fungicide and which fungicide, weather conditions (rain and wind), and application method (ground vs. air). The general guideline is the following: 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 US and Canada
Stripe rust have 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 US and Canada is almost like the most widespread situation in 2010.
Figure 1. Active pustules (left) of susceptible reaction in comparison with moderate level of resistance (right) of wheat to stripe rust. If more than 20% of the leaf tissue is dead, significant yield loss can occur although less than the similar percentage of leaf areas covered by rust pustules. Under the current heaven rust inoculum and low-temperature conditions, it is beneficial to spray with fungicides on wheat varieties with ratings 4 and above (moderately resistant to susceptible). Susceptible wheat plants sprayed with a fungicide after infection can appear like the picture on the right. Spray fungicides before infection can reduce dead stripes.