Wheat powdery mildew and foliar N concentrations as influenced by N fertilization and belowground interactions with intercropped faba bean

Wheat (Triticum aestivum L.) production by intercropping with faba bean (Vicia faba L.) has increased in popularity but is often associated with severe wheat powdery mildew (Blumeria graminis (DC.) Speer). Very little is known about the effects of below- and aboveground interspecific interactions on wheat nitrogen (N) nutrition and occurrence of wheat powdery mildew. A greenhouse pot experiment examined four N application rates and three belowground partition types (plastic film, nylon mesh partition or no partition) to study N nutrition and interactions between wheat and faba bean growing together. A field experiment investigated three N application rates and growth of wheat in monoculture and intercropped with faba bean with or without belowground plastic film partitions between wheat and faba bean. Disease incidence (DI) and disease severity index (DSI) were assessed at flowering stage and wheat leaves were sampled and analyzed for N. Foliar N was enhanced substantially by N addition in greenhouse and field conditions and also by belowground interactions (no partition compared with plastic film partition) in the pot experiment (all P < 0.001). There was a significant synergistic effect between N rate and belowground interactions on the enhancement of wheat N uptake (P < 0.01) in the pot experiment. DI and DSI of mildew increased markedly with increasing N rate in both experiments (all P < 0.001). In the pot experiment DI and DSI showed no marked differences among belowground partitions (both P > 0.05) but belowground interactions had different effects under different N rates, limiting disease occurrence under 0, 0.1 and 0.2 g N kg−1 soil but promoting disease with 0.05 g added N kg−1 soil. In the field experiment DI and DSI showed no significant differences between wheat monoculture and intercropping (both P > 0.05). However, the contributions of below- and aboveground interactions to disease control were different under different N rates, with interspecific root interactions increasing DI and DSI under different N rates and aboveground interactions increasing DI and DSI under zero-N application but decreasing DI and DSI at 150 and 300 kg N ha−1. The data suggest that the microclimate in the field and biological control mechanisms due to belowground interactions in wheat–faba bean associations may influence the incidence and severity of wheat powdery mildew.