The proportion of resistant hosts in mixtures should be biased towards the resistance with the lowest breaking cost.

Current agricultural practices facilitate emergence and spread of plant diseases through the wide use of monocultures. Host mixtures are a promising alternative for sustainable plant disease control. Their effectiveness can be partly explained by priming-induced cross-protection among plants. Priming occurs when plants are challenged with non-infective pathogen genotypes, resulting in increased resistance to subsequent infections by infective pathogen genotypes. We developed an epidemiological model to explore how mixing two distinct resistant varieties can reduce disease prevalence. We considered a pathogen population composed of three genotypes infecting either one or both varieties. We found that host mixtures should not contain an equal proportion of resistant plants, but a biased ratio (e.g. 80 : 20) to minimize disease prevalence. Counter-intuitively, the optimal ratio of resistant varieties should contain a lower proportion of the costliest resistance for the pathogen to break. This benefit is amplified by priming. This strategy also prevents the invasion of pathogens breaking all resistances. Author summary: This study addresses the optimal design of mixtures of resistant hosts to reduce disease prevalence, and prevent the emergence and invasion of multi-virulent pathogen genotypes. Specifically, we investigated how pathogen mediated plant-plant interaction, through immune priming of host defences, influences the optimal proportion of each resistant host in such mixtures. We thus designed a mathematical model explicitly accounting for immune priming in mixtures of two resistant plant varieties. We showed, through analysis and simulation, that the optimal ratio is not 50:50, as commonly done in practice, but should be biased towards the variety that is the least costly for the pathogen to break. We also showed that this bias depends on immune priming effectiveness, and is enhanced when virulence costs are high. This somewhat counter-intuitive outcome finds its explanation in the complex interplay of ecological and physiological mechanisms acting in cultivar mixtures, and is of major relevance for the application of mixtures in agricultural practice. This model therefore provides new clues to best manage and exploit plant biodiversity for sustainable plant health. It also provides new insights into how host heterogeneity and immunity can prevent the evolutionary emergence of pathogens capable of breaking several resistances. [ABSTRACT FROM AUTHOR]