Fungicidal Activity of Caproate Produced by Clostridium sp. strain E801, a Bacterium Isolated from Cocopeat Medium Subjected to Anaerobic Soil Disinfestation

Abstract

Anaerobic soil disinfestation (ASD) consists of the application of labile organic materials to soil, flooding, and covering the soil surface with plastic film. Anaerobic soil disinfestation is a widely used ecofriendly alternative to chemical fumigation for eliminating soil-borne plant pathogens. However, the exact mode of action of ASD has not been elucidated. In particular, the potential role of anaerobic soil bacteria in disinfestation is unclear. In this study, we isolated a predominant bacterium designated as strain E801 from cocopeat medium after laboratory-scale ASD with ethanol as the carbon source. The strain was closely related with Clostridium kluyveri, and fermentatively produced butyrate and caproate from ethanol and acetate. Interestingly, the culture supernatant of strain E801 strongly suppressed the growth of Fusarium oxysporum f. sp. lycopersici (Fol) in a pH-dependent manner. Among the volatile fatty acids produced by E801, only caproate showed significant growth suppression at pHs below 5.5. In addition, caproate eliminated Fol conidia completely at pHs 5.5 and 5.0 and suppressed Fol growth even at a low temperature (15 °C). Furthermore, cocopeat medium amended with caproate eliminated Fol conidia completely within 6 days. These results suggest that caproate is one of the key disinfestation factors in ethanol-based ASD and that the direct application of caproate to soil could be a promising strategy for rapid and stable soil disinfestation.