Quantifying the relative importance of disease-suppressive mechanisms in species mixtures: a case study of late blight in strip-intercropped potato

Numerous studies have reported disease suppression in intercropping systems, attributing it to mechanisms such as host dilution, microclimate modification, barrier effect, and induced resistance. However, the relative contributions of mechanisms to altered disease dynamics remains unclear. We combined field experiments and mechanistic modelling to quantify the importance of these mechanisms in suppressing Phytophthora infestans in potato intercropped with faba bean, ryegrass, or maize. Field data was used to estimate effects of disease-suppressive mechanisms on various disease processes. These were integrated into a dynamic microclimate-dependent epidemiological simulation model of late blight to predict the progression of disease severity, and the individual contribution of mechanisms. Even small differences (1-3%) in relative humidity accumulated to significantly impact disease severity. The model most accurately predicted disease suppression only when host dilution, microclimate modification and barrier effect were combined, suggesting that each contributes substantially. Individual mechanisms varied in strength across companion crops and sometimes counteracted (particularly microclimate modification and barrier effect), but their combined effects consistently reduced disease. This study provides a novel framework to disentangle and quantify the contribution of disease-suppressive mechanisms in intercropping systems, enhancing our understanding of disease suppression in species mixtures, to help design cropping systems less reliant on chemical protection.