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Assessment of Six Models of Host-Pathogen Interaction in Horizontal Pathosystems. M. L. Carson, Assistant professor, Plant Science Department, South Dakota State University, Brookings 57007; Phytopathology 77:241-246. Accepted for publication 16 July 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-241.

The following six genetic models of host-pathogen interaction in horizontal pathosystems were evaluated: the interaction for resistance, interaction for susceptibility, Parlevliet and Zadok’s addition, Fleming and Person’s additive. Fleming and Person’s multiplicative, and a multiplicative interactive model. Evaluations were made by examining the relationship between pathogen aggressiveness and genetic variation for disease reaction in host F₂ populations, based on a two-locus system controlling host resistance and pathogen aggressiveness. Models were also evaluated for their usefulness in detection of significant cultivar × isolate interactions in the analysis of variance approach for detecting specificity. Host F₂ genetic variances increased as the level of pathogen aggressiveness increased with interaction for susceptibility, multiplicative and interactive-multiplicative models, remained constant with the additive model, and decreased with interaction for resistance and addition models. Cultivar × isolate interactions in analyses of variance would be difficult to detect in real experiments and are not necessarily indicators of gene-for-gene specificity or stability of resistance. The relationship between host genetic variance for disease reaction and level of pathogen aggressiveness appears to be a viable method for assessing stability of resistance and the type of host-pathogen interaction model that might apply to a particular pathosystem. An example of the analysis indicates that resistance of wheat to Pyrenophora tritici-repentis is stable and that the most aggressive pathogen isolates should be used in screening for resistance.