Physiologic specialization of Puccinia triticina on wheat in the United States in 2003

Collections of Puccinia triticina were obtained from rust infected wheat leaves by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, Southeast, California, and the Pacific Northwest, in order to determine the virulence of the wheat leaf rust fungus in 2003. Single uredinial isolates (580 in total) were derived from the wheat leaf rust collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes Lrl, Lr2a, Lr2c, Lr3, Lr9, Lr/6, Lr24, Lr26. Lr3ka, Lr11, Lrl7, Lr30, LrB, LrlO, Lr14a, and Lr18. In the United States in 2003, 52 virulence phenotypes of P. triticina were found. Virulence phenotype MBDS, which has been selected by virulence to resistance gene Lr17, was the most common phenotype in the United States. MBDS was found in the Southeast, Great Plains, the Ohio Valley, and California. Virulence phenotype THBJ, which has been selected by virulence to genes Lr16 and Lr26, was the second most common phenotype, and was found in the southern and northern central Great Plains region. Phenotype MCDS, which has been selected by virulence to genes Lr17 and Lr26, was the third most common phenotype and occurred in the same regions as MBDS. The use of wheat cultivars with leaf rust seedling resistance genes has selected leaf rust phenotypes with virulence to genes Lr9, Lr16, Lrl7, Lr24, and Lr26. The population of P. triticina in the United States is highly diverse for virulence phenotypes, which will continue to present a challenge for the development of wheat cultivars with effective durable resistance.