Long-term suppression of Pythium abappressorium induced by Brassica juncea seed meal amendment is biologically mediated

Evidence indicates that seed meal (SM) of Brassica juncea is an effective biofumigant against Pythium spp., an important biological component contributing to apple replant disease. However, the ability of this seed meal to provide disease control even after termination of allyl isothiocyanate (AITC) emission suggested that unidentified mechanisms are also involved in suppression of certain pathogens in B. juncea SM-amended soil. When soils were infested with Pythium abappressorium 2–12 weeks after SM was applied, disease suppression was consistently observed in SM-treated soil. Bagging of soil for the initial 48 h after SM application, to simulate tarping of soil in the field, enhanced disease control. Application of SM either as coarse or fine particles produced similar effects on disease suppression. B. juncea SM amendment also suppressed the proliferation of P. abappressorium observed in Brassica napus SM-treated soils at a time point well after AITC emission from soils was no longer detected. Pasteurization of SM-amended soil eliminated soil suppressiveness toward this pathogen, demonstrating the important contribution of the soil microbiota to the disease control attained in AITC evacuated soil. Terminal-Restriction Fragment Length Polymorphism profiles obtained for 18S rDNA from fungal communities associated with SM-amended and non-amended soil demonstrated distinct variation in terms of composition. Visible changes in fungal community composition in SM-treated soils were also observed, and analyses indicated the preferential proliferation of Trichoderma spp. in SM-treated soils. These findings suggest that modification of the resident fungal community in SM-amended soil may contribute to the observed long-term suppressiveness of B. juncea SM-amended soils toward apple root infection by P. abappressorium.