Fungal pathogen accumulation at the expense of plant-beneficial fungi as a consequence of consecutive peanut monoculturing

Peanut yield and quality are seriously compromised by consecutive monoculturing in the red soil region of southern China. Soil fungi are, however, also critical to the ecological functioning of soils and plant health and were thus the study subject here. Using 454 pyrosequencing, the entire soil fungal communities of a field where peanut had been consecutively cultured for five years and another field, where peanut had been consecutively cultured for at least 20 years, and a control field with only a single peanut crop after a lay period were compared. Fungal richness, community composition, and relative taxon abundances in soil were compared among the fields and sampling times, the latter corresponding to the pod-bearing and the pod-maturing stages. Eight hundred fungal operational taxonomic units at 97% ITS sequence identity were found among 194,783 sequence reads derived from 18 separate soil samples. Members of the phylum Ascomycota strongly dominated the soil fungal communities and putative pathogens, such as Fusarium oxysporum, Leptosphaerulina australis, Phoma sp., and Bionectria ochroleuca showed higher relative abundances in the fields where peanut was consecutively monocultured, compared to the control field, at the expense of putatively plant-beneficial fungal groups, such as Trichoderma sp., a glomeromycotan fungus, and Mortierella elongata. The results suggest that the accumulations of fungal pathogen loads at the expense of plant-beneficial fungi in the soil appear likely explanations for yield declines as a consequence of consecutive peanut cultivation.