Stochastic simulation of the spread of race-specific and race-nonspecific aerial fungal pathogens in cultivar mixtures

The spread of race-specific and -nonspecific fungal pathogens in cultivar m ixtures over space and time was simulated using an individual-based, spatia lly explicit stochastic model. The spatial spread of disease was simulated using a half-Cauchy distribution. The effects of five simulation variables on the effectiveness of cultivar mixtures in reducing disease development w ere investigated. These simulation variables were the sporulation rate, the median spore dispersal distance, the probabilities of cross-infection amon g hosts and pathogen races, the proportion of host plants that were complet ely susceptible (or, in the case of race-specific pathogens, the numbers of mixture components) and the spatial arrangement of the mixture components. Disease dynamics were summarized by the rate parameters of logistic equati ons and by the area under the disease progress curve (AUDPC) of incidence a nd severity. The potential reduction in disease development in cultivar mix tures, compared with pure cultures, was considerable. Mixtures were more ef fective in reducing race-specific pathogens than race-nonspecific pathogens . For both types of pathogen, most variation in logit-transformed mixture e fficacy was due to the main effects of the simulation variables. For race-n onspecific pathogens, the performance of mixtures was influenced mainly by the proportion of plants that were susceptible and by the spatial arrangeme nt of the two mixture components. For race-specific pathogens, the performa nce of mixtures was determined mainly by the number and the spatial arrange ment of mixture components. The smaller the homogeneous genotype area, the greater the mixture efficacy. Higher sporulation rate decreased mixture eff icacy. Planting the mixture components in square blocks was more effective in reducing disease than planting in strips. For race-nonspecific pathogens , increasing the proportion of susceptible plants decreased the mixture eff icacy. For race-specific pathogens, disease in mixtures decreased with incr easing numbers of mixture components. The effect on the mixture efficacy of increasing cross-infection probability from 0 to 0.25 was generally small. For the AUDPC-based efficacy of disease severity, the effects of median sp ore dispersal distance were also very large: the shorter the median spore d ispersal distance, the less effective the mixture.