By Tim Murray, Extension Plant Pathologist
Dr. Xianming Chen, USDA-ARS Pullman, released his most recent Stripe Rust Update May 5, 2016. In it he notes that stripe rust has continued to spread and is developing quickly across the region, including the high rainfall producing areas of the Palouse. Many fields have already received one or more applications of fungicide. I’ve received several calls wondering what to do now as the winter crop ranges from boot to heading or flowering, and the near-term forecast calls for the weather to remain favorable for stripe rust development.
First and foremost, it’s important to continue scouting fields and look for rust, regardless of the resistance reaction of the variety planted. You may find no rust (resistant varieties), brownish-colored lesions (resistant reaction) with few or no spores present (moderately resistant varieties) or actively sporulating lesions where the yellow spores rub-off on your finger (moderately susceptible or susceptible varieties).
Regardless of the variety, if you find actively sporulating lesions on 5% or more of the leaves, and the plants are not yet to Feeke’s 10.5.4 (anthesis complete-kernels watery ripe), and it’s been 3 weeks or more since the last fungicide application, consider another fungicide application. Although some winter varieties appear more susceptible this year than last (e.g. ORCF-102, Xerpha, Keldin, Curiosity CL+ and WB Junction), there has been no detectable change in the races of stripe rust yet. These differences are due to the early start of the rust epidemic this year and weather conditions preventing adult-plant resistance from becoming fully effective.
The same recommendations apply to spring wheat. Follow-up with regular scouting of fields and spray a fungicide when active rust reaches 5% in moderately susceptible or susceptible varieties. A higher percentage of rust (20%) can be tolerated in moderately resistant varieties before fungicide application.
Make sure to read and follow label directions for all fungicides. Note that only 8 oz of propiconazole can be applied to a field in one season and that it must not be applied after Feeke’s stage 10.5.4 (kernels watery ripe). Some fungicides, such as tebuconazole, have a 30-day preharvest restriction for application, but applications after watery ripe kernel development have diminishing returns in terms of yield response.
Finally, it should be noted that Feeke’s 10.5.1 begins when the anthers appear (first in the center of the head) and ends when the anthers are present from the top to the bottom of the head (Feeke’s 10.5.3). Depending on weather, flowering may last for a week; less with warmer and more with cooler temperatures.
Stripe rust ratings on most commercially grown varieties can be found using the Variety Selection Tool. More information on stripe rust, including NCERA Fungicide 2015, is available in under Foliar Fungal Diseases in the Disease Resources section of this website.