2.1.4
EARLY SPRING SURVIVAL OF WHEAT LEAF RUST IN THE CENTRAL GREAT PLAINS OF THE USA

M EVERSMAYER and CL KRAMER

USDA-ARS, Department of Plant Pathology, 4007 Throckmorton Hall, and Division of Biology, Kansas State University, Manhattan, KS 66506 USA

Background and objectives
Severe leaf rust epidemics, which result in economic yield reductions in the Great Plains wheat-producing region of the USA are usually initiated by inoculum that has survived in the local field from the previous wheat crop until early spring. Survival of wheat leaf rust between Great Plains wheat crops depends on amount of moisture and temperatures occurring between harvest and emergence of the new wheat crop in the fall. Abnormally high temperatures and low moisture may prevent establishment of volunteer wheat after harvest, which reduces or prevents inoculum survival from the previous crop. Winter survival of inoculum on volunteer wheat or the autumn-planted crop depends largely on sufficient moisture for infection and the protection of infected leaves from sudden changes in sub-freezing temperatures [1]. Urediniospores from infections that have survived in local fields provide inoculum for wheat leaf rust epidemics in the central winter wheat-producing region of northern Texas to southern Nebraska, USA, in 8 out of 10 years. Our previous studies of wheat leaf rust epidemics in Kansas found that economic reductions in yield of greater than 2% were always measured when inoculum survived the winter and early spring within local fields [2].

Materials and methods

Stepwise multiple regression was used to identify weather variables that explained the most variation in inoculum survival at the final tiller development wheat growth stage at several sites in the central Great Plains during 1980-97. Inoculum survival was recorded on a 0-9 scale with 0 indicating no survival and 9 indicating inoculum on all plants in the field. Independent variables used in development of the models were daily deviations from the 10-year average of temperature, moisture and snow cover data for each site. Moving 10-year averages were determined for each daily weather variable or variable calculated from daily observations. For example, the 10-year average minimum temperature for 1985 was determined using data for 1975-84 and the 1986 10-year average minimum temperature used data for 1976-85. The dependent variable was the amount (0-9) of wheat leaf rust inoculum observed on or prior to final tiller development for each site and year combination.

Results and conclusions
Models were constructed to forecast inoculum survival from data collected prior to autumn wheat planting, the beginning of winter dormancy and final tiller development of the wheat crop. Seventy per cent of the observed occurrences of leaf rust overwintering were forecast by models constructed with data prior to planting. Eighty per cent of the time, overwintering could be forecast by models constructed with data prior to the wheat entering winter dormancy. Models constructed with data prior to final tiller development in the spring forecast overwintering of leaf rust 95% of the time. Early warning of potential overwintering of wheat leaf rust inoculum will allow production decisions to be made or control measures to be employed prior to and at planting, and as the wheat crop breaks dormancy in the spring. Results from these studies will be used in developing forecasts of leaf rust severities and crop loss.

References


1. Burleigh, JR, Schulze, AA, Eversmayer, MG 1969. Plant Disease Reporter 53, 947-53
2. Eversmayer, MG, Kramer, CL and Browder, LE 1988. Plant Diseases 1074-76