Wheat (Triticum aestivum L) is grown throughout the grasslands from southern Mexico into the prairie provinces of Canada, a distance of nearly 4200 km. The total area seeded to wheat varies considerably each year; however, from 28 to 32 million ha are planted in the Great Plains of the United States alone. Generally in the central Great Plains, an area from central Texas through central Nebraska, 15 million ha are seeded to winter wheat each year. A wide range of environmental conditions exist throughout this area that may affect the development and final severity of wheat leaf rust (caused by Puccinia triticina L), stripe rust (caused by P. striiformis), and stem rust (caused by P. graminis Pers. f. sp tritici) epidemics and the subsequent reduction in wheat yields. Variation in severity of rust epidemics in this area depends on differences in crop maturity at the time of infection by primary inoculum, host resistance used, and environmental conditions. The interrelationships among time, host, pathogen and environment are complex, and studying the interactions is very difficult. Historically, cultivars with new or different leaf rust resistance genes become ineffective after several years of large-scale production within the Great Plains, and then cultivars carrying new or different resistance genes must be developed and released into production. This is the typical "boom and bust" cycle of the cereal rust resistance genes in the central Great Plains.

A backnard-elimination, multiple-regression analysis was used to determine which metedrological factors significantly explained the variation measured in concentrations of allergenic spore forms 1.5 m above ground level. Airspora were collected for three years (19761978) with volumetric samplers. Adequate moisture is probably the most important variable in spore production. In the imperfect fungi, once the spores are produced, release is often influenced by wind velocity. In the Ascomycetes, radiation, minimum humidity, changes in humidity, and minimum wind velocity were all directly correlated with airborne ascospores, while in the Basidiomycetes, precipitation was the most important variable directly correlated with basidiospores in the atmosphere.

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