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Development of American Isolates of Puccinia graminis f. sp. tritici on an Artificial Medium. W. R. Bushnell, Plant Physiologist, Crops Research Division, ARS, USDA, Cooperative Rust Laboratory, University of Minnesota, St. Paul 55101; D. M. Stewart, Research Plant Pathologist, Plant Protection Division, ARS, USDA, Cooperative Rust Laboratory, University of Minnesota, St. Paul 55101. Phytopathology 61:376-379. Accepted for publication 10 November 1970. DOI: 10.1094/Phyto-61-376.

Uredospores of 25 diverse American isolates of Puccinia graminis f. sp. tritici were seeded in dense populations on agar containing peptone, glucose, and mineral salts, then incubated under cool, moist conditions. Twenty isolates formed white mycelial mats, indicating development beyond the germ-tube stage. Eleven isolates grew slowly (6-41 µ/day) in subcultures made from the stromata. Growth rates usually declined through successive monthly subculturings until growth ceased, but the growth rates of two isolates increased to 42-64 µ/day by the sixth subculture. These two isolates have continued to grow until the present time (9 months or more). The results indicate that American isolates of the stem rust fungus vary widely in culturability, and that present techniques are not adequate for long-term culture of most of these biotypes.