The genetical control of incomplete forms of resistance to Erysiphe graminis in spring barley

Eight spring barley lines exhibiting incomplete resistance to powdery mildew in field trials were crossed and back-crossed to a common susceptible parent to produce the basic generations (P₁ P₂, F₁ F₂, B₂ B₂) required for a genetical investigation. By means of joint scaling tests, disease assessments on adult plants in a field trial were used to estimate the genetic parameters contributing to resistance. Lines with major genes, which appeared partially resistant due to the presence of corresponding virulence, were characterised by the presence of a significant dominance component in the analysis. Partially resistant lines which either lacked identifiable major genes or with major genes that were ineffective in the trial, but with good background resistance, could also be distinguished. Significant additive and epistatic effects were detected in all crosses. The prospect of enhancing partial resistance through hybridisation and selection is discussed.