Wheat stripe rust (Puccinia striiformis West.) epidemics are confined predominantly to the Pacific Northwest in the U. S. A. because of climate. This disease was frequently reported until the late 1930's and then virtually absent until the late 1950's. Since the severe epidemic in 1961, stripe rust has been frequently severe on winter wheat and has caused losses in susceptible cultivars in many years. Because of the unusual history of stripe rust in this region, the possibility that climate variability affected the pattern of rust occurrence was investigated. Meteorological data for seven locations in Oregon, Washington, and Idaho were analyzed. In 1961-1974 for the Columbia Basin locations, January and February temperatures averaged 1.20° C higher than during the period 1935-1960; however, April temperatures averaged 1.28° C lower in 1961-1974 than during the earlier period. Monthly precipitation averages have not varied more than 12.7 mm in any month. Between 1961-1974, December snowfall almost doubled over that in 1935-1960; snowfall in February decreased over 50% from the earlier period. Data was computed on a seasonal basis since 1901 and considered in respect to stripe rust epidemics. Since 1961, above-normal winter and below-normal spring temperatures have increased the frequency and severity of stripe rust epidemics in the Pacific Northwest. The direction of temperature and precipitation trends varied with the time period considered. How the climate variability which has occurred may have affected winter wheat growth and yields is postulated. Studies such as this should be useful to researchers modelling crop-yields, agronomists evaluating results from field experiments and to researchers studying fluctuations in pest populations. © 1979 D. Reidel Publishing Company.
