Studies on anaerobic soil disinfestation (ASD), a non-chemical alternative to soil fumigants for controlling plant diseases caused by soilborne pathogens, have shown that ASD increases populations of beneficial microorganisms with activity against plant pathogens, including species of Trichoderma and Streptomyces, which are known to parasitize sclerotia of Sclerotium rolfsii. However, studies on ASD effectiveness paired with inoculation of Trichoderma and Streptomyces are lacking. This study compared the effects of ASD, soil incorporation of antagonists [Trichoderma asperellum, T. harzianum (RootShield (R)), Streptomyces griseoviridis (Mycostop (R)) and T. harzianum + S. griseoviridis], and ASD + soil incorporation of antagonists at the initiation of ASD treatment, on sclerotial germination and colonization. The effect of ASD and soil incorporation of antagonists on abundance of Trichoderma spp., actinomycetes, and Bacillus spp. in soil were assessed. In contrast to the negative effect of ASD on sclerotial germination, we observed positive or no effect of ASD alone on populations of beneficial microorganisms evaluated. ASD treatment that included addition of T. asperellum or T. harzianum did not increase colonization of sclerotia by Trichoderma spp. compared to ASD alone, whereas ASD treatment with inoculation of S. griseoviridis did increase sclerotia colonization by actinomycetes. Indigenous soil actinomycete populations were suppressed by incorporation of T. asperellum and T. harzianum during ASD treatment. Soil abundance of T. harzianum was increased by ASD treatment, and further increased by T. harzianum inoculation at ASD treatment initiation. Inoculation of T. asperellum, T. harzianum, or S. griseoviridis at ASD treatment did not further reduce germination of sclerotia of S. rolfsii, indicating that addition of these microorganisms to soil prior to ASD treatment is unlikely to improve ASD treatment efficacy against pathogens with resistant sclerotial survival structures.
