Studies on Bacterial Populations Concomitant with Sclerotium rolfsii sclerotia in Flooded Soil, as Estimated by 16S rRNA Gene, PCR-DGGE and Sequence Analyses

Objective: PCR-DGGE was used to examine the bacterial communities associated with sclerotia of Sclerotium rolfsii, the causal agent of soybean stem rot. Methodology and Results: Fungal sclerotia were buried in soil amended with organic matter and incubated under flooded conditions for 15 or 30 days in a greenhouse and in the field.  The viability and associated bacterial communities of the recovered sclerotia were investigated. In samples from soil amended with rice straw or wheat bran and flooded for 30 days, DGGE band patterns revealed the greatest bacterial diversity.  In the greenhouse, for example, the diversity index from organic amendment under 30 day flooding was greater than 1.80, whereas the index for other treatments was less than 1.5. In the field experiment, this pattern was similar. The diversity index derived from DGGE band patterns had a negative relationship with sclerotial viability or disease incidence. The nucleotide sequences of the DGGE bands revealed that members of the Clostridiaceae were dominant in samples that had been flooded for 30 days, whereas Oxalobacteraceae, Nocardiaceae, and Actinomycetaceae were major groups under unflooded conditions. Conclusion: This is the first report of a soil bacterial flora associated with S. rolfsii sclerotia under flooded conditions.