Three principles for more informative virulence surveys for wheat rusts
To inform breeders and growers of important changes in virulence and to facilitate development and deployment of resistant cultivars, isolates of wheat rust fungi are routinely evaluated on seedlings of a set of differential wheat lines containing different resistant genes. However, the methods used to evaluate and report virulence changes in most regions of the world seem inadequate for accomplishing these goals and could be improved by adherence to three principles. Firstly, for each region, the resistance genes in the set of differentials should match the effective genes in contemporary cultivars and breeding lines. Most differential sets contain several resistance genes that have been ineffective for decades and do not contain genes found in cultivars and breeding lines. Given the importance of genes for race-specific adult-plant resistance, these should be included in differential sets. Secondly, intermediate reactions on differential lines that had been highly resistant are important warnings of gradual increases in virulence. Naming races requires isolates to be either virulent or avirulent on each line in a fixed set of differentials and is a hindrance to identifying gradual changes in virulence on currently effective genes. Utilizing virulence formulae with a designation for intermediate virulence (e.g. parentheses around the gene or differential) seems to be a simple solution for both documenting partial virulence and for easily changing differentials to match genes in cultivars and breeding lines. Thirdly, the method for evaluating virulence against a particular differential should predict the result of that host-pathogen interaction in the field. Growth stage and environmental conditions are important for expression of some resistance genes, and all currently effective genes are not likely to be expressed under the same conditions. Following these principles will make virulence surveys more predictive of important changes in the field and thereby contribute to more effective management of rust diseases.