Present Knowledge and Theories Concerning Durable Resistance

Summary

Examination of the additive and specific interaction models proposed by Parlevliet and Zadoks (1977) led to the conclusion that the comparison made of these models by these authors was not valid, and that their conclusions were therefore not supported by their analysis. Examination of the so-called Person model, which assumes polygenes that interact nonspecifically, showed that this model predicts constant ranking of both hosts and pathogens, and that at least two factors, (viz. “phenotypic damping,” and opposition of selective forces operating in the two interacting populations), would contribute to stability. Research on the genetics of pathogenicity of Ustilago hordei toward cultivated barley, also described in this paper, showed that for this system pathogenicity is determined by major genes, by polygenes, and by interaction between major genes and polygenes. It was concluded that an understanding of durable resistance must be based on detailed genetic knowledge of host resistance as well as of pathogenicity, and in particular of how polygenes and major genes of host and pathogen interact. It may be that no single model or theory will turn out to be universally applicable.