The assessment and significance of pathogenic variability in Puccinia striiformis in breeding for resistance to stripe (yellow) rust: Australian and international studies

Stripe (yellow) rust, caused by Puccinia striiformis, continues to cause crop losses in several regions of the world. National and international control strategies focussing on breeding resistant cultivars are dependent on relevant infonnation concerning the nature and extent of pathogenic variation. Detailed studies in Australia have shown a progressive evolution of pathotypes which have, in some instances, led to epidemics and caused significant problems for commercial wheat production. Recent changes in the pathogen population have caused potential problems for the barley industry .While studies have been undertaken in certain locations for varying periods of time, the continuing need for specialist facilities, experienced staff and common di.fferential testers has generally resulted in discontinuous data sets. In order to overcome these difficulties, and provide a basis for regular data collection, a simplified method based on field assessment of near isogenic lines (NILs) has been evaluated in several international locations. The NILs, which are based on a selection of the susceptible spring wheat cultivar Avocet, have allowed the effectiveness of a range of single resistance genes to be detennined. Data arising from this international project has provided evidence for regional differences in pathogenic variability between populations of P. striiformis f.sp. tritici. For example virulence for Yr 1 was common in the eastern districts of Central Asia, China and certain locations in India. However, virulence for Yr9 was more widespread in diverse locations including the Middle East (Iraq, Turkey, Syria), China, Africa (Uganda), Australasia (New Zealand) and South and Central America (Chile, Ecuador, Mexico). Avirulence for certain genes, including Yr5,Yr15 and Yr18, suggests potential usefulness of these genes in breeding programs. However, virulence for the fonner two resistance genes reported elsewhere would preclude their adoption in some breeding programs. The paper will describe these studies, and discuss the use of the data in breeding programs aimed at the release of stripe rust resistant wheats.