Virulence of Puccinia triticina on wheat in the United States from 1996 to1998

Isolates of Puccinia triticina were obtained from wheat leaf collections ma de by cooperators throughout the United States and from surveys of wheat fi elds and nurseries in the Great Plains, Ohio Valley, and Gulf Coast states in 1996, 1997, and 1998. Virulence-avirulence phenotypes were determined on 14 host lines that are near-isogenic for leaf rust resistance. We found 31 phenotypes among 277 single uredinial isolates in 1996, 56 phenotypes amon g 989 isolates in 1997, and 43 phenotypes among 989 isolates in 1998. As in previous surveys, regional race distribution patterns showed that the cent ral United States is a single epidemiological unit distinct from the easter n United States. The distinctive racial composition of collections from the southeast, northeast, and Ohio Valley indicate that populations of P. trit icina in those areas are not closely connected, suggesting that epidemics o riginate from localized overwintering sources.