Physiologic specialization of Puccinia triticina on wheat in the United States in 2005

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 River Valley, southeast, California, and Washington State, in order to determine the virulence of the wheat leaf rust population in 2005. Single uredinial isolates (797 in total) were derived from the collections and tested for virulence phenotype on lines of Thatcher wheat that are near-isogenic for leaf rust resistance genes LrI, Lr2a, Lr2c, Lr3a, Lr9, Lr16, Lr24, Lr26, Lr3ka, Lr11, Lr17a, Lr30, LrB, LH10, Lr14a, Lr18, Lr21, Lr28, and winter wheat lines with genes Lr41 and Lr42. In the United States in 2005, 72 virulence phenotypes of P. triticina were found. Virulence phenotype TDBGH, selected by virulence to resistance gene Lr24, was the most common phenotype in the United States, and was found throughout the Great Plains region. Virulence phenotype MCDSB with virulence to Lr17a and Lr26 was the second most common phenotype and was found widely in the wheat growing regions of the United States. Virulence phenotype MFPSC, which has virulence to Lr17a, Lr24, and Lr26, was the third most common phenotype, and was found in the Ohio Valley region, the Great Plains, and California. The highly diverse population of P. triticina in the United States will continue to present a challenge for the development of wheat cultivars with effective durable resistance to leaf rust.