FIELD-RESISTANCE TO RUSSIAN WHEAT APHID IN BARLEY .1. SYMPTOM EXPRESSION

Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), is a serious pest of wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). This research was conducted to identify barley genotypes resistant to RWA under field conditions. Spring barley genotypes were evaluated during summer 1989 (109 genotypes) and winter 1990 (111 genotypes) at El Batan, Mexico. Hill-plots of 8 to 10 seeds were infested with 90 to 110 greenhouse-reared RWA per hill. Hills were visually rated on a 1 to 6 scale (1 = no symptoms) for RWA damage at tillering (Zadoks 10 to 29), jointing (Zadoks 30 to 35), and post-heading (Zadoks 59 to 65). Genotypes differed significantly (P < 0.01) in reaction to RWA at individual scoring dates, and averaged over all scoring dates. Fifteen genotypes were classified as resistant (score < 2.51) based on overall mean score; seven of these were resistant at all scoring dates. The latter included selections from the crosses SMA1/SB401, API/CM67//AGER, ASE/2CM//B.7.6.B.B, GLORIA/COME, CI1255/ARAMIR// GLORIA/3/CI2325/CI12225//BOY*2/3*SURB, ECT/6/M/N// V/4/USB//13914/C/3/SMA/5/SMA, and SD729/POR/3/APM/ ATHS//GVA/4/ORE. Significant (P < 0.01) genotype X crop cycle and genotype X growth stage interactions suggest the possibility of improving detection of material with useful levels of resistance by screening in the environment where the material will ultimately be grown, and observing plants at various stages during crop growth. Despite the complication of interaction effects, field screening using artificially infested hill-plots was effective in identifying barley genotypes that expressed RWA resistance across a broad range of growing conditions and crop development.