Evolution of a pathogen population in host mixtures: Rate of emergence of complex races

The development of a pathogen population in a crop varietal mixture was studied with an epidemic simulator based on the model EPIMUL. The pathogen population was composed of simple races, able to develop on only one genetic component of the mixture, and a complex race, which developed on all mixture components. The complex race was modelled with a very low initial frequency compared to simple races, to simulate the emergence of a complex race in the field. Several successive epidemics were simulated as if the pathogen population reproduced on the same plot for several years. The development of the complex race on the simulated plot was either focal or uniform. The effects of the cost of virulence, of density dependence and of differential adaptation to host genetic background on the simple race-complex race competition were studied. Experimentally measured values of the cost of virulence and differential adaptation were incorporated into the model, and both factors were shown to greatly reduce the increase in frequency of the complex race over time. Density dependance also influenced race competition, but mainly for high values of the parameter. Our results suggest that the cost of virulence is probably not the only mechanism that may influence the simple race-complex race competition in host mixtures. In our simulations, differences in the spatial distribution of the initial inoculum between parasite races led to large differences in their final frequencies. Thus, more investigations, including randomized disease distributions, would be of interest to judge the potential importance of spatial effects in the field.