Stripe Rust Forecast and Update
On March 1, I was checking wheat fields in Whitman, Columbia, Walla Walla, Benton, and Franklin counties in Washington, mostly south of Highway 26. Plants ranged from two-leaf stage to early jointing (Feekes 1-5), depending upon planting time and region. In most wheat fields, plants were still in winter dormancy and did not grown much compared to growth stages in early November, but became uniform especially in late planted fields. No rust was found in any of the checked fields. There was no obvious winter injury or other diseases.
Prediction and Management Suggestion
Using a series of forecast models based on various weather parameters from December to February (Sharma-Poudyal and Chen, Phytopathology 101:544-554, 2011), the new prediction of potential yield loss caused by stripe rust on susceptible winter wheat varieties is 35±5%, dropping from the 47% forecasted based on only the December weather. This level is within a moderate epidemic category. The prediction is based on historical yield loss and weather data of the Palouse region and is applicable for most wheat growing regions in the Pacific Northwest, except western Washington, western Oregon, and southern Idaho as these regions have very different weather patterns. With the moderate level of damage potential, susceptible and moderately susceptible varieties may need to be sprayed with fungicide (possibly just once) and resistant and moderately resistant varieties may not need spray. Please keep in mind that the current prediction can be realized only if the weather conditions from now throughout the remaining growing season are normal (past long-term averages). If May and June are unusually cool and wet, just like in 2010, stripe rust would be severer than the current prediction. On the other hand, if the weather conditions from April to June are drier and from May to June hotter than normal, stripe rust would be lower than the current prediction. Nevertheless, based on the current prediction and field observations, stripe rust will definitely not be as severe as 2011 (but still have possibilities to be as severe as in 2010), applications of fungicides may not be necessary until you see stripe rust (5-10% incidence) in your fields. Checking fields should be started from later March in eastern Oregon and southern-central Washington (Horse Heaven Hills, Connell areas, and Walla Walla) and from middle April in areas further north and east in Washington and Idaho. In western Washington, stripe rust should be visible as always in this time of the year. Application of fungicides together with herbicides in fields grown with susceptible and moderately susceptible varieties should be a standard procedure for maximizing profit every year in western Washington.
Current Stripe Rust Situations in other States
Stripe rust was first reported in Arkansas on January 27, in Mississippi on January 30, in California in the last week of February, and in Texas on March 1. Stripe rust has been developing and spreading quickly in Arkansas and Mississippi. The recent storms and tornadoes would have spread stripe rust spores to infect wheat fields in states further north and east.