Rhizosphere Fungal Community Dynamics Associated with Rehmannia glutinosa Replant Disease in a Consecutive Monoculture Regime

Consecutive monoculture of Rehmannia glutinosa in the same field leads to a severe decline in both quality and yield of tuberous roots, the most useful part in traditional Chinese medicine. Fungi are an important and diverse group of microorganisms in the soil ecosystem and play crucial roles in soil health. In this study, high-throughput pyrosequencing of internal transcribed spacer 2 ribosomal DNA amplicons was applied to gain insight into how consecutive monoculture practice influence and stimulate R. glutinosa rhizosphere and bulk soil fungal communities. The results from nonmetric multidimensional scaling ordination and clustering analysis revealed distinctive differences between rhizosphere and bulk soil fungal communities. However, longer-term monocultured bulk soils were more similar to the rhizosphere soils in comparison with the shorter-term monocultured bulk soils. Moreover, consecutive monoculture caused a gradual shift in the composition and structure of the soil fungal community. The cultivation of this plant led to the appearance of some exclusive operational taxonomic units in rhizosphere or bulk soils that were assigned to the genera Fusarium, Rhizoctonia, and so on. Furthermore, the sum of the relative abundance of species of Fusarium, Cylindrocarpon, and Gibberella (belonging to the family Nectriaceae); Rhizoctonia, Thanatephorus, and Ceratobasidium (belonging to the family Ceratobasidiaceae); and Lectera and Plectosporium (belonging to the family Plectosphaerellaceae) was significantly higher in consecutively monocultured (CM) than in newly planted (NP) soil in both rhizosphere and bulk soils. In particular, Fusarium abundance was significantly higher in CM than in NP in the rhizosphere, and higher in rhizosphere soils than in bulk soils for each treatment. A pathogenicity test showed that both Fusarium strains isolated were pathogenic to R. glutinosa seedlings. In addition, the culture filtrate and mycotoxins produced by Fusarium oxysporum significantly repressed the growth of the antagonistic bacterium, Pseudomonas aeruginosa. In conclusion, consecutive monoculture of R. glutinosa restructured the fungal communities in both rhizosphere and bulk soils but bulk effects developed more slowly over time in comparison with rhizosphere effects. Furthermore, microbial interactions might lead to a reduction in the abundance of beneficial microbes.