Arbuscular mycorrhiza protects soybean plants against Macrophomina phaseolina even under nitrogen fertilization

The capacity of arbuscular mycorrhizal fungi (AMF) to alleviate the negative effects incited by root pathogens in a range of plant hosts has been established. On the other hand, accumulated evidence also shows that fertilization practices can negatively impact AMF. Nevertheless, the interaction between AMF, pathogens and fertilizers, especially nitrogen (N) fertilizers, has not been previously reported. In this work, the effect of nitrogen on both the severity of the pathogen Macrophomina phaseolina (charcoal root rot) and the protection by the arbuscular mycorrhiza fungi (AMF) Rhizophagus intraradices was investigated in greenhouse experiments using soybean (Glycine max) as a host. The treatments were two levels of N (0 and 92 kg of urea ha−1), inoculation and non-inoculation with the AMF, and infection and non-infection with the pathogen. Soybean was harvested at R4 phenological stage (completed pod formation). Plant biomass, numbers of pods and leaves, plant height, root length, greenness index, mycorrhizal colonization and disease severity were measured. Pathogen infection reduced soybean biomass and negatively affected the greenness index, but co-inoculation with AMF improved these parameters. Nitrogen fertilization reduced AMF colonization but not arbuscules percentage. N fertilization increased disease severity but mycorrhizal symbiosis was able to reduce it. These results demonstrate that severity of charcoal root-rot disease in N fertilized soybean can be reduced by AMF inoculation. The implication of these results is that N fertilization could increase the risk of diseases in soybean but mycorrhiza could contribute to soybean charcoal root rot control even if the crop is under N fertilization.