Association of Two Unknown Pythium spp. without Forming Sexual Structures on Root Rot of Hydroponically Grown Welsh onion

Aims: Aim of this study was to elucidate association of Pythium isolates without forming sexual structures on root rot of hydroponically grown Welsh onion, as well as to clarify their taxonomic positions. Place and Duration of Study: Commercial hydroponic greenhouses in Hiroshima Prefecture, Hiroshima Prefectural Technology Research Institute, and Osaka Metropolitan University, between March 2020 and March 2021. Methodology: Total 2,966 isolates of Pythium-like organs were isolated from the plant roots during the survey period. All the isolates didn’t form any sexual organs, and were separated into three groups based on colony patterns on agar media such as cornmeal agar. Among of them, 236 isolates were equally selected from each three groups. Since two of the three groups were thought to be first records on Welsh onion, 17 representative isolates from the two groups were examined on morphology, hyphal growth response to temperatures and sequences of the ribosomal internal transcribed spacer (ITS) regions and mitochondrial COI. Inoculation tests were performed on Welsh onion seedlings under a hydroponic condition by using one representative isolate from each group. Results: The three groups were identified as Pythium Cluster B2a sp., B1d sp. and Clade A sp. on sequences of ITS and COI, respectively. Pythium isolates belonged to Pythium Cluster B1d and Clade A showed similar morphology and hyphal growth response to temperatures each other. Inoculation tests confirmed that both representative isolates from two groups cause roots rot of hydroponically grown Welsh onion 14 days after their inoculation. Pythium Cluster B1d showed stronger pathogenicity than Pythium Clade A. Conclusion: The present study firstly demonstrated that Pythium Cluster B1d sp. and Clade A sp. which don’t form any sexual structures associate root rot of hydroponically grown Welsh onion.