Wheat Diseases in the Great Plains
Viral and fungal diseases are important in winter wheat production in the Great Plains. The arthropod transmitted viral diseases include wheat streak mosaic virus and its associated mite-vectored diseases, and barley yellow dwarf virus. Important soil borne fungal and viral pathogens occur in the Great Plains. Fusarium species and Cochliobolus sativa cause Fusarium root rot and common root rot, respectively. Fusarium and common root rot are limited to dry soils and, as such, are more common in the High Plains than in eastern parts of the Great Plains. Take-all disease (caused by Gaeumannomyces graminis var. tritici), Rhizoctonia root rot (caused by several Rhizoctinia species), and Pythium root rot (caused by several Pythium species) are favored by wet soils. However, these diseases are not restricted to high rainfall areas, and yields may be significantly affected during short periods of wet soils during early crop establishment and growth. Take-all, Fusarium root rots, and Pythium root rots occur throughout the Great Plains. Soilborne wheat mosaic is widespread but is most important in the eastern Great Plains because its fungal vector prefers wet soils. These problems are influenced by climate, soil type, and agronomic practices. Diversified crop rotations are not considered to be effective against these pathogens because they can survive for several years in the soil.
Leaf rusts, stem rusts, and powdery mildew are important leaf diseases of wheat in the Great Plains. The rusts are managed mainly with resistant varieties and, to a lesser extent, with fungicides. However, races of rust fungi that can overcome resistance of ten develop, so diversification within a region is important. Powdery mildew is similar to rusts in that infection spreads by spores released in the environment. Resistant varieties are widely used. No role for crop diversification in rust and powdery mildew management has been demonstrated. Diversification might be expected to reduce their importance because fewer susceptible plants would be available to contract the disease within a region, thus reducing the chance for widespread outbreaks to develop (see also Chapter 9-"Disease Management of Wheat").