Characterization of Stripe Rust Resistance in Wheat Lines with Resistance Gene Yr17 and Implications for Evaluating Resistance and Virulence

Abstract

Stripe rust, caused by Puccinia striiformis f. sp. tritici, has been the most important foliar wheat disease in south central United States since 2000 when a new strain of the pathogen emerged. The resistance gene Yr17 was used by many breeding programs to develop resistant cultivars. Although Yr17 was classified as a seedling (all-stage) resistance gene conferring a low infection type, seedlings with Yr17 frequently had intermediate to high infection types when inoculated with isolates that caused little or no disease on adult plants of the same wheat lines. The objectives of this study were to determine how to best evaluate Yr17 resistance in wheat lines and to determine which factors made seedling tests involving Yr17 so variable. Stripe rust reactions on wheat seedlings with Yr17 were influenced by temperature, wheat genotype, pathogen isolate, and the leaf (first or second) used to assess the seedling reaction. The most critical factors for accurately evaluating Yr17 reactions at the seedling stage were to avoid night temperatures below 12°C, to use the first leaf to assess the seedling reaction, to use multiple differentials with Yr17 and known avirulent, partially virulent and virulent isolates as controls, and to recognize that intermediate infection types likely represent a level of partial virulence in the pathogen that is insufficient to cause disease on adult plants in the field.

Yellow Rust Epidemics Worldwide Were Caused by Pathogen Races from Divergent Genetic Lineages

“…* Represents the clonal NW European lineage refering to many additional races prevalent in Europe up to 2010 .** Reference isolates from PstS1 provided by Gene Milus and extensively described in Milus et al (2015a,b) .*** Synonymous to “Triticale Aggressive” in Hovmøller et al (2016); v24/Avr24 could not be assessed due to avirulence on Avocet S .…”

“…** Reference isolates from PstS1 provided by Gene Milus and extensively described in Milus et al (2015a,b) .…”

“…The results were based on a set of samples from important wheat varieties and breeding lines in different geographical regions and years. Comparable sample sizes were considered for most of the regions, except North America where isolates consisted of important references from previous studies (Milus et al, 2015a,b). The study did not include the Australian population, which is known to be dominated by races emerging from an incursion of a particular race from NW-Europe in 1978 (Wellings and McIntosh, 1990) and PstS1 related races (Wellings, 2007; Hovmøller et al, 2008).…”

Double dose efficiency of the yellow rust resistance gene Yr17 in bread wheat lines

“…They were grown in the field at INRA Le Rheu, Brittany, France, and inoculated with the yellow rust pathotype 237E141 AV17 as described in Dedryver et al () and Paillard et al (). A field resistance assessment showed that all plants carrying Yr17 were highly resistant at adult plant stages (Dedryver et al, , Paillard et al, and Milus, Lee, & Brown‐Guedira, ). Resistant plants and those with no obviously distinctive traits conferred by 6N v (height, pale‐green leaf, long peduncle) were selected.…”

“…Milus et al () showed that yellow rust reactions on wheat seedlings with Yr17 were influenced by temperature, wheat genotype and pathogen isolate. Seedlings with Yr17 could have intermediate to high infection types when inoculated with isolates that caused little or no disease in adult plants of the same wheat lines.…”