Update from Kirk Broders, Assistant Professor in the Department of Bioagricultural Sciences and Pest Management, Colorado State University.
It happened earlier than I expected, but we received our first confirmed report of stripe rust in eastern Colorado today. We had samples sent in from the Prospect Valley area just northeast of Denver, and we have identified stripe rust on these samples. This represents a significant ‘jump’ for the pathogen to make from southeastern Kansas or Oklahoma all the way to central Colorado. The field was planted with ‘Ripper’, which is a highly susceptible variety to stripe rust, so any potential inoculum in the area could have lead to these infections. I would advise growers in this area to scout field for the presence of stripe rust, particularly if you have planted Ripper or another highly susceptible variety.
Stripe rust will spread most readily in 50-60 degree temperatures especially during wet rainy weather, which is what eastern Colorado will likely see over the course of the next 1-2 weeks. The disease spread is still likely up to 65 degrees and is possible at 70 degrees. Windblown spores will germinate at night when the temperatures and leaf moisture condition are right. Spore formation is stopped when daytime temperatures exceed 80 degrees.
All treatments are preventative and none are very good at curing stripe rust infections. It is therefore very important to apply stripe rust fungicides before the first sign of infection. In previous years, the timing between first signs of the rust and widespread infection were only 2-5 days, but with stripe rust arriving this early in the year the incubation period will likely be longer. Finally, CSU and other University research on rust fungicides have not found any benefits from treatments in the absence of rust. That is why it will be important to scout fields and be aware of the presence or absence of stripe rust in your specific region.
Susceptible wheat varieties under irrigation are the most at risk. There should not be a significant impact on resistant wheat varieties, however there were reports last year of some resistant varieties showing signs of stripe rust due to the extreme inoculum pressure, so keep this in mind. The 2009 Wheat Decisions handbook includes variety information on stripe rust susceptibility or resistance: http://www.extsoilcrop.colostate.edu/CropVar/documents/winterwheat/wheatreport_2009.pdf
As always if you identify stripe or leaf rust in any of your fields please send samples into the CSU diagnostic lab as well as an email/photo to me. This will allow me to make a positive identification and notify the larger community regarding the current distribution of the pathogen in the state in order to inform growers of the potential need for a fungicide application.