RECURRENT SELECTION FOR ADAPTATION OF ERYSIPHE-GRAMINIS F SP HORDEI TO PARTIAL RESISTANCE AND THE EFFECT OF ENVIRONMENT ON EXPRESSION OF PARTIAL RESISTANCE OF BARLEY

Five isolates of powdery mildew were subjected to recurrent selection on partially resistant barley cultivars and tested at intervals for adaptation to overcome partial resistance. A trend towards adaptation to the recurrent host was observed in infection frequency data, but biomass per colony, measured using sterol cell-wall determinations and enzyme linked immunosorbent assay, was not greatly affected. The expression of partial resistance was examined after seedling growth under two different environmental regimes. Significant interactions with both overall resistance expression and latent period were detected using infection frequency data, and the effect were both cultivar and isolate dependent. Variability between single colony derivatives of an isolate, which had previously shown adaptation to a partially resistant cultivar, was detected in infection frequency data indicating instability and a non-nuclear component to adaptation.