Biotype Differences for Resistance to Russian Wheat Aphid in Barley

Russian wheat aphid [RWA; Diuraphis noxia (Mordvilko)] is a worldwide insect pest of barley (Hordeum vulgare L.), causing crop losses each year. Previously identified resistant barley lines do not show variable reactions to the eight United States RWA biotypes identified by wheat reactions. However, additional RWA isolates have been identified outside the United States. The objective of this research was to determine whether additional RWA biotypes that impact resistance in barley exist and whether resistance to these biotypes is present in the resistant barley germplasm and cultivars that have been developed by the USDA-ARS. A total of 166 barley lines were rated for chlorosis and leaf rolling in response to infestation with RWA United States' biotype 1 and to individual isolates from Hungary, Chile, Mexico, and Iran. Each barley line was genotyped for 5912 single nucleotide polymorphism (SNP) markers, and the combined data were used for association mapping of loci associated with resistance to each RWA isolate. A total of 40 loci were associated with resistance to the five RWA isolates. Biotype differences were evident, with only 23 lines showing resistance to all five biotypes. Loci with large effects on mean chlorosis and rolling differences were found for resistance to biotypes from the United States, Hungary, and Chile, while the loci for resistance to biotypes from Mexico and Iran had small effects on these traits, which may pose problems for developing resistant germplasm. The 23 lines resistant to all biotypes included 10 unadapted germplasm accessions and 13 improved germplasm lines and provide useful germplasm for developing new barley cultivars with resistance to multiple RWA biotypes.