|Leonard, Kurt - USDA/ARS/CDL RETIRED|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 25, 2001
Publication Date: N/A
Interpretive Summary: Leaf rust is a potentially devastating disease of wheat that is controlled in the United States mainly by resistant varieties. Many genes for leaf rust resistance are available to wheat breeders, but populations of the leaf rust fungus contain a variety of virulent races. Wheat breeders need to know which resistance genes will be effective against leaf rust races that occur where their varieties will be grown. In 1999 leaf rust caused yield losses of 4% in MN and ND and 3.4% in KS. Collections of the wheat leaf rust fungus were made throughout the United States as part of an ongoing program to monitor leaf rust races. Each of 1180 rust isolates collected in 1999 was tested for virulence on wheat lines with 14 different genes for resistance. Leaf rust races with virulence to overcome five genes for leaf rust resistance commonly used in US wheat varieties increased to potentially damaging levels from 1996 to 1999. In particular, losses to leaf rust in spring wheat in the upper Midwest were greater than at any time in the last 20 years. The increased leaf rust can be attributed to emergency breeding for scab resistance using unadapted wheat parents that have high susceptibility to leaf rust. 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 triticina were obtained from wheat leaf collections made by cooperators throughout the United States and from surveys of wheat fields and nurseries in the Great Plains, Ohio Valley, and Gulf Coast states in 1999. Virulence/avirulence phenotypes were determined on 14 host lines that are near isogenic for leaf rust resistance. We found 58 phenotypes among 1180 isolates in 1999. As in previous surveys, regional race distribution patterns showed 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 ofP. triticina in those areas are not closely connected, suggesting epidemics originate from localized overwintering sources.