Modeling Winter and Early Spring Survival ofPuccinia reconditain Wheat Nurseries During 1980 to 1993

Survival of Puccinia recondita inoculum between wheat crops is critical to the occurrence of severe leaf rust epidemics, which result in economic yield reductions in the Great Plains wheat-producing region of the U.S. Meteorological variables occurring prior to spring green-up of the wheat crop during 1980 to 1993 at Manhattan, KS, were used to model survival of inoculum throughout the winter and early spring in wheat nurseries. Stepwise multiple regression techniques were used to determine those weather variables that explained the most variation in levels of inoculum surviving on 15 March. Inoculum levels were recorded on a 0 to 9 scale with 0 indicating no inoculum survival and 9 indicating inoculum on all plants. Daily maximum and minimum temperatures, fungal temperature equivalence function, precipitation and snow cover, cumulative precipitation and fungal temperature function, and daily deviations from the 10-year average of those variables were averaged for 10-day periods prior to a date of prediction and used as independent variables. Models that explained 99% of the variation in overwintering with five or six variables were developed for the fifteenth of each month from December through March. Models for December, January, and February used five of the same variables, but the minimum temperature deviation used in the December model was replaced by the January rainfall deviation in the January and February models. The model for March used a different set of temperature variables and included daily deviations in snow cover for December and February to explain a significant portion of the overwintering of P. recondita inoculum.