• ammi analysis;
  • bacterial wilt of forage grasses;
  • biplot;
  • disease resistance;
  • Italian ryegrass;
  • race-specificity

Bacterial wilt of forage grasses, caused by the pathogen Xanthomonas translucens pv. graminis (Xtg), is a major disease of forage grasses such as Italian ryegrass (Lolium multiflorum). The plant genotype-bacterial isolate interaction was analysed to elucidate the existence of race-specific responses and to assist the identification of plant disease resistance genes. In a greenhouse experiment, 62 selected plant genotypes were artificially inoculated with six different bacterial isolates. Significant differences in resistance were observed among Lmultiflorum genotypes (< 0·001) and in virulence (intensity of disease symptoms) among Xtg isolates (< 0·001) using the area under the disease progress curve (AUDPC). No significant genotype-isolate interaction (> 0·05) could be observed using linear regression modelling. However, additive main effects and multiplicative interaction effects (ammi) analysis revealed five genotypes which did not cluster close to the origin of the biplot, indicating specific interactions between these genotypes and some bacterial isolates. Simple sequence repeat (SSR) markers were used to identify marker-resistance associations using the same plant genotypes and bacterial isolates. The SSR marker NFA027 located on linkage group (LG) 5 was significantly associated with bacterial wilt resistance across all six bacterial isolates and explained up to 37·4% of the total variance of AUDPC values. Neither the inoculation experiment nor the SSR analyses revealed major host genotype-pathogen isolate interactions, thus suggesting that Xtg resistance, observed so far, is effective across a broad range of different bacterial isolates and plant genotypes.