Host‐mediated interactions between pathogen genotypes

Barley c ltivars with different resistance genes were inoculated with various sequences of virulent and non‐virulent isolates of Erysiphe graminis f.sp. hordei. The resistance induced in incompatible reactions decreased infection by virulent isolates while, conversely, the degree of susceptibility induced in compatible reactions was sufficient to increase the ability of normally non‐virulent isolates to infect a resistant host. The net effect of interactions was always overall reduction of conidial production when compared with inoculation of a virulent isolate alone. Each interaction between isolates was dependent on the host cultivar, the pathogen isolates used and on the inoculation sequence. Often, induced resistance was most obvious when a nonvirulent isolate was inoculated prior to a virulent isolate. Induced susceptibility was usually more apparent when the virulent isolate was inoculated first. However, on the cultivar Wing, induced susceptibility was more apparent following prior inoculation of a non‐virulent isolate. On the cultivar Hassan, prior inoculation with a virulent isolate sometimes induced susceptibility, but in some cases more susceptibility was induced by prior inoculation with a non‐virulent isolate. In the latter instances, although relatively little infection developed, the majority of the spores produced were those of the normally non‐virulent isolate. There was some evidence of non‐virulent isolates acquiring the ability to infect normally incompatible hosts. This effect disappeared after several generations in the absence of the virulent isolate.