Assessment of Six Models of Host-Pathogen Interaction in Horizontal Pathosystems

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Abstract
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. Evaluation 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 .times. 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 .times. 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.