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 w ere found among 728 single uredinial isolates on 14 host lines that ar e 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 c entral United States is a single epidemiological unit distinct from th e eastern United States. The distinctive racial composition of collect ions from the Southeast, Northeast, and Ohio Valley indicate that popu lations of P. r. tritici in those areas are discrete, suggesting epide mics originate from localized overwintering sources. Although collecti ons from nurseries were not significantly more diverse than collection s from fields, they did differ substantially in some areas. In the Nor theast, the racial composition of nursery collections showed little re lationship to that in field collections.