PULLMAN, Wash. — Wheat stripe rust is developing very rapidly on Zak and several other spring wheat varieties in Eastern Washington, according to Kim Kidwell, a spring wheat breeder at Washington State University and Xianming Chen, a plant pathologist with the USDA Agricultural Research Service.
Kidwell said the fungal disease has been reported in the higher rainfall region of eastern Washington along the Idaho border.
Rapid development of the epidemic is due to rustfavorable weather, and possibly to new rust strains that circumvent disease resistance in Zak.
The disease shows up as orange pimplelike pustules arranged in fine lines or stripes along wheat leaf veins, explained Diana Roberts, WSU Cooperative Extension, Spokane and Lincoln counties. A magnifying glass may be required to see them. She said that general yellowing of lower leaves without evidence of the pustules cannot be attributed to the rust pathogen.
It is essential to positively identify the rust before spraying. Stripe rust generally occurs on lower leaves first, and may rapidly infect upper leaves under favorable weather conditions. Dry or windy conditions should slow the process. A heavy infection on a leaf will impede photosynthesis and grain fill. It is important to protect the flag leaf, which provides about 50 percent of plant yield.
Kidwell said that if stripe rust is found on 10 percent of any leaf on 10 percent of the plants in a stand, consider spraying with fungicide. Based on rust levels in the field and weather forecast for the next seven days, it is estimated that stripe rust could cause about 30 percent to 60 percent yield loss on Zak and other susceptible spring wheat varieties if fungicide is not used immediately, according to Chen.
Stripe rust also reduces grain quality by lowering test weight.
Chen said that three registered fungicides Tilt, Quadris and Stratego can be applied to protect the crops from further rust damage. Based on crop stage, effectiveness of the fungicides, and economics, only one application of fungicide is generally recommended.
Tilt, which was previously labeled for use until the ligule of the flag leaf emerges, can be applied up to late head emergence under an emergency label. The ligule is the outgrowth on the inner side of the wheat leaf at the point where the sheath joins the blade. Tilt should be applied at the rate of 4 fluid ounces per acre.
Quadris can be applied up to late head emergence at a rate of 6.2 fluid ounces per acre. A crop oil concentrate adjuvant may be added at one percent volume per volume to optimize efficacy.
Stratego can be applied, at a rate of 10 fluid ounces per acre, until the ligule of the flag leaf emerges. Based on the current label, do not apply Stratego after this growth stage and do not apply within 35 days of harvest.
The researchers say that although the wheat varieties Alpowa, Penawawa, and Wawawi have some level of hightemperature, adultplant resistance and while the resistance is still effective against the rust strains present this year, they may suffer some yield losses if the disease pressure is high.
In a study conducted with Alpowa last year, application of fungicide increased grain yield by 10 percent to 20 percent.
By assessing the yield potential of their stand compared to the cost of application, growers should be able to determine if it will be profitable to spray fungicide on Alpowa.
“We have seen low infection levels of stripe rust on Scarlet, but relatively little activity on Westbred 926 in experimental field plots to date,” Kidwell said in an email to extension agents and others. “If stripe rust activity is detected on these varieties in commercial fields, consider spraying fungicide if the yield potential justifies the cost of application.
“Please check all of your spring wheat fields for stripe rust infection. More varieties in our field plots are becoming infected with stripe rust every day. This pathogen is spreading very quickly based on the current weather patterns, and we expect stripe rust infection levels to increase in commercial production fields over the next 7 days.
“With the exception of Westbred 470 and a few new varieties, such as Columbia 1, winter wheat varieties are generally resistant to stripe rust and do not require fungicide application. Even if stripe rust is detected on winter wheat crops at this time, most plants are past the growth stage up to which fungicide application is recommended by label.”
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