Epidemiology of wheat leaf and stem rust in the central Great Plains of the USA

Wheat (Triticum aestivum L) is grown throughout the grasslands from souther n Mexico into the prairie provinces of Canada, a distance of nearly 4200 km . The total area seeded to wheat varies considerably each year; however, fr om 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 t hrough 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 ru st (caused by 19 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 p rimary inoculum, host resistance used, and environmental conditions. The in terrelationships among time, host, pathogen and environment are complex, an d studying the interactions is very difficult. Historically, cultivars with new or different leaf rust resistance genes become ineffective after sever al years of large-scale production within the Great Plains, and then cultiv ars carrying new or different resistance genes must be developed and releas ed into production. This is the typical "boom and bust" cycle of the cereal rust resistance genes in the central Great Plains.