The soybean cyst nematode (SCN) is a major yield-limiting pest of soybean. In this study, experiments were conducted to examine the diversity of parasitic fungi from SCN associated with disease-suppressive soil fields in Northeast China. Soil samples were collected from three fields under different rotation systems that were established in 1991: (1) a continuous long-term cropping field with soybean (SSSS) that had been shown to be SCN-suppressive, (2) cycles of three-year rotation with corn, soybean, and wheat (WCS), and (3) continuous cropping field with three-year cycles of two years soybean and one-year corn (SSC). In the traditional method result, cyst densities of SCN declined as increase of parasitic fungi, and the percentage of parasitic fungi associated with cyst of SCN was higher in SSSS field than other two fields. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) also showed that parasitic fungi of SCN were also increased in SSSS field, compared with the other two fields. Principal component analysis based on PCR-DGGE data revealed that fungal communities on cysts could be divided into three groups: one group occurred in SSSS, and the other two groups were in WCS and SSC fields, respectively. Long-term cropping with soybean monoculture in the black soil field might increase parasitic fungi of SCN. These fungal communities may play an important role in the ecological suppression of SCN in disease-suppressive soil.

Experiments were conducted to study the inhibitor factors present in suppressive soil that suppressed the soybean cyst nematode (SCN) in the black soil fields with 21 years continuous cropping of soybean (SSSS) and 3 years continuous cropping of soybean followed by 1 year corn (SSSC). Compared with the SSSC field, the densities of SCN significantly (P < 0.05) declined in SSSS field. In a soil microcosm with suppressive soil, the population of SCN decreased after the addition of pimaricin (fungal inhibitor) and streptomycin (bacterial inhibitor), but the population of SCN declined more significantly by adding streptomycin to the suppressive soil than adding pimaricin. Fusarium equiseti, Pochonia chlamydosporia and Purpureocillium lilacinum were isolated from cysts, eggs and J2 of SCN and soil in SSSS and SSC field. The population density of these parasitic fungi was significantly (P < 0.05) higher in SSSS field than in SSSC field. The density of P. chlamydosporia was higher than other fungi. Real-time PCR was used to test genomic DNA of P. chlamydosporia in SSSS and SSC field soil. The mass of genomic DNA and the population density of P. chlamydosporia were significantly (P < 0.05) higher in SSSS field than in SSSC field. This study has revealed that fungi, especially P. chlamydosporia, may play an important role in the suppression of SCN population in the suppressive soil.

Disease-suppressive soils confer fitness advantages to plants after a disease outbreak due to the subsequent assembly of protective microbiota in natural environments. However, the role of ecological effects on the assemblage of a protective soil microbiome is largely elusive. In this study, we investigated the composition of parasitic microbes and their relationships with soybean cyst nematodes in suppressive soil. The results showed that parasitic microbial assembly along soybean cyst nematodes was shaped predominantly by the density of soybean cyst nematodes. We also found soybean continuous cropping increased the number of parasitic microbes of soybean cyst nematodes with the order of Ss > Sr > Sc, while it decreased the population of soybean cyst nematodes, resulting in a natural decline in the number of soybean cyst nematodes. These findings indicate that the population of soybean cyst nematodes accumulated parasitic microorganisms against this soil-borne disease under soybean long-term continuous cropping. Moreover, the metabolic activity of cyst parasitic microbes was increased by two years of continuous cropping (Sc) of soybean, and total carbon and total nitrogen of soil were the main impact factors in this short-term continuous cropping for metabolic patterns of the cyst parasitic microbes. In summary, the results highlight that the interaction of plants and disease shape the soil microbiome, recruit a group of disease resistance-inducing microbes, and modulate their beneficial traits to protect the plant.