Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/1994
Publication Date: N/A
Interpretive Summary: Leaf rust, a potentially devastating disease of wheat, is controlled by resistant varieties grown in the United States. Many genes for leaf rust resistance are available to wheat breeders, but populations of the leaf rust fungus contain a variety of virulent races. Therefore, wheat breeders need to know what races are present in their regions each year in order to know which resistance genes will be effective. Collections of the wheat leaf rust fungus were made throughout the United States in 1992, as part of ongoing monitoring of leaf rust races. Each rust isolate was tested for virulence on wheat lines with 14 different genes for resistance. Increase of virulence to several specific resistance genes was noted in some parts of the United States. The distinctive racial composition of collections from some areas indicate that populations of rust in these areas are discrete, suggesting localized epidemics from local overwintering sources. Nationally, losses in yield from leaf rust in winter wheat were estimated at 2.2% in 1990, 3.3% in 1991 and 4.8% in 1992. The trends in importance of leaf rust and changes in prevalence of races in different areas of the United States will be used by wheat breeders to choose sources of resistance for new varieties and by wheat pathologists to assess the need for additional rust control measures in their states.
Technical Abstract: 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 isogenic for leaf rust resistance. The frequencies of virulence to lines with Lr24 and Lr 26 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. recondita f. sp. 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.