Facilitation between intercropped species increases micronutrient acquisition and controls rust disease on maize

Context or Problem: Global food security is threatened by plant disease, and crop diversification often promotes productivity through reduced disease and facilitation for increased nutrient acquisition. However, whether such facilitation is a factor in disease resistance is unknown. Objective or Research Question: Investigate how crop diversity affects crop productivity and rust disease on maize, and whether competition or facilitation between crop species correspond with resistance. Methods: Five irrigated intercropping experiments with different fertilization and crop combinations were conducted at two sites for four years. Productivity (542 data points) and disease severity (27150 data points) of maize monoculture, maize - legume and maize - non-legume intercropping were compared. A meta-analysis of literature was performed to confirm the broader applicability of results from these field experiments. Results: Legume-based intercropping increased the aboveground biomass of intercropped maize by 8% and grain yields by 10% in comparison to non-legume-based intercropping. Disease severity on maize intercropped with legumes was reduced by 45% and 48%, compared to monocultures of maize and maize intercropped with non-legumes, respectively. Moreover, as interactions among intercrops became more facilitative, the concentrations of zinc (Zn), copper (Cu) and iron (Fe) in maize increased, and these increases were highly correlated with decreasing disease severity. The global meta-analysis was consistent with our field experiments, as lower disease severity was associated with greater intensity of interspecific facilitation or with lower intensity of interspecific competition. Conclusions or Significance: Lower disease severity was closely related to enhanced acquisition of nutrients that can enhance the resistance to crop diseases, driven by stronger interspecific facilitative effects in intercropping systems. Facilitative effects on maize was increased by the identity of leguminous companion crop species, and was increased by sufficient irrigation, but reduced by applications of nitrogen and phosphorus fertilizers. Implications: Our findings identify a novel facilitative mechanism in general and advance the understanding of the facilitative mechanisms that underly disease control through crop diversification.