Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/23/2012
Publication Date: 5/10/2012
Citation: Weerakoon, D.M., Reardon, C.L., Paulitz, T.C., Izzo, A., Mazzola, M. 2012. Long-term suppression of Pythium abappressorium induced by Brassica juncea seed meal amendment is biologically mediated. Soil Biology and Biochemistry. 51:44-52. Interpretive Summary: Organic soil amendments often have been promoted as a means to control soilborne plant diseases. However, the effective use of such an environmentally sensitive method of disease control has been impeded by a lack of understanding concerning the means by which amendments provide disease control. Tissues from plants belonging to the Brassicaceae have been promoted as a soil amendment for the control of soilborne plant diseases due to their production of glucosinolates, which yield anti-microbial compounds upon hydrolysis. In this study, Brassica juncea seed meal, a waste residue resulting from oil seed extraction was evaluated for the ability to control Pythium abappressorium, a soilborne pathogens that contribute to development of apple replant disease. In the case of Brassica juncea seed meal, initial disease control is directly related to the release of the chemical allyl isothiocyanate (AITC), a product resulting from the hydrolysis of plant glucosinolates. However, long-term control of Pythium in response to B. juncea seed meal amendment require the activity of the microbial community present in the treated soil. B. juncea seed meal application induced changes in the fungal community present in the treated soils, and specifically resulted in increased recovery of Trichoderma, a fungus known to provide effective control of Pythium. The changes in fungal community composition, and proliferation of Trichoderma, was driven by the production of AITC. That is, when production of AITC was abolished or minimized, changes in fungal community were not obtained and disease control was not achieved. Thus, the long-term control of Pythium in response to this seed meal is AITC dependent but mediated by the native soil biology.
Technical Abstract: Evidence indicates that seed meal 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 render disease suppression even after termination of allyl isothiocyanate (AITC) emission suggested that unidentified mechanisms are also involved in suppression of certain pathogens in B. juncea seed meal (SM)-amended soil. When soils were infested with Pythium abappressorium 2 to 12 weeks after SM was applied, disease suppression was consistently observed in SM-treated soil. Bagging of soil during the initial 48 h after SM application, to simulate tarping of soil in the field, significantly increased disease control. Application of SM either as coarse or fine particles produced similar effects on disease suppression. B. juncea SM amendment also demonstrated the capacity to suppress the proliferation of P. abappressorium observed in Brassica napus SM-treated soils at a time point well after AITC emission from soils is 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.