Pathogenic fungi accumulation rather than direct autotoxicity is responsible for the root-rot disease of foxtail millet under continuous monoculture regimes

Abstract

Foxtail millet is a dietary staple cultivated in arid and semiarid regions worldwide but its sustainable cultivation is strongly restricted by continuous cropping obstacles. Here, we compared the performance of foxtail millet, rhizosphere soil fungi communities under non-continuous cropping, and two and eight years of continuous monocultures (C0, C2, and C8, respectively) to explore the underlying mechanisms. The emergence rates and yield of foxtail millet decreased under continuous monoculture, and the magnitude increased with years of the monoculture. The C8 soil slurry alone and in combination with bactericide (Bronopol) significantly suppressed the emergence rates and root length of foxtail millet, whereas the presence of fungicide (Captan) almost entirely attenuated the suppressive effects, indicating that fungi, but not autotoxicity, are responsible for the negative effects of the continuous cropping on the performance of foxtail millet. Eight-year of monoculture decreased the relative abundance of the fungal genera Acaulium, Gymnoascus, Mortierella, Solicoccozyma, and Pseudombrophila, stimulated the relative abundance of the fungal genera Fusarium, Acremonium, and Cephalotrichum. An F. oxysporum strain, YDSi-3, was isolated from the C8 rhizosphere soil, which induced root-rot disease in foxtail millet. The concentrations of phenolic acids, especially cinnamic acid, significantly increased in the C8 rhizosphere soil. The application of cinnamic acid largely increased the abundance of F. oxysporum in C0 soils. Overall, our findings suggest that the negative effects of continuous cropping on foxtail millet may be attributed to pathogenic fungal accumulation because of the phenolic-acid enrichment in the rhizosphere.