VIRULENCE AND DIVERSITY OF PUCCINIA-RECONDITA F SP TRITICI IN THE UNITED-STATES IN 1992

Isolates of Puccinia recondita f. sp. tritici were obtained from wheat leaf collections made by cooperators throughout the United States and from cereal rust field surveys of the Great Plains, Ohio Valley, and Gulf Coast states in 1992. Fifty-two virulence/avirulence phenotypes were found among 728 single uredinial isolates on 14 host lines that are near-isogenic for leaf rust resistance. The frequencies of virulence to lines with Lr24 and Lr26 during 1992 were greater than in previous years. Regional race distribution patterns again suggested that the central United States is a single epidemiological unit distinct from the eastern United States. The distinctive racial composition of collections from the Southeast, Northeast, and Ohio Valley indicate that populations of P. r. tritici in those areas are discrete, suggesting epidemics originate from localized overwintering sources. Although collections from nurseries were not significantly more diverse than collections from fields, they did differ substantially in some areas. In the Northeast, the racial composition of nursery collections showed little relationship to that in field collections.