Introduction

1. Introduction Evidence indicates that seed meals of several Brassicaceous species may be effective biofumigants against soil-borne pathogens (Brown and Morra, 1997; Matthiessen and Kirkegaard, 2006; Hoagland et al., 2008). For organic growers, relatively few choices exist for the control of such pathogens. With increasing interest in Brassicaceous species as biofuel sources, the seed meal, which is a by-product of the oil extraction process, will likely increase in availability. Upon wetting, the seed meal of Oriental mustard (Brassica juncea) releases allyl isothiocyanate (AITC), which is volatile and which has pesticidal properties. Brassica juncea seed meal (BSM), therefore, may be a good candidate for fumigation if its use does not damage the crop. Our objective was to examine BSM for its potential as a biofumigant in organically-certified greenhouse potting mix. We hypothesized that the effectiveness of BSM as a fumigant would depend on how much time elapsed after incorporating it into a potting mix that was contaminated with Pythium spp., which causes damping-off in spinach. Brassica juncea seed meal as a fumigant in organic greenhouse productionStewart Higgins1, Lynne Carpenter-Boggs1, Mark Mazzola2, Timothy Paulitz2, Jeffery Smith2, Catherine Crosby3.1BIOAg Program, CSANR; 2USDA-ARS; 3Department of Crop and Soil Sciences, WSU • Results • After the initial seeding, spinach emergence was consistently higher in the +BSM pots than in the control pots (Figs. 1a and 1b). • Spinach seedling mortality was higher in control pots than in the +BSM pots at all incubation times except one (Figs. 2a and 2b). • In the control pots, viable Pythium decreased during the first 3 days of incubation, and then increased thereafter (Fig. 3a). • In the +BSM pots, after time zero, no Pythium CFUs were detected (Fig. 3a). • Total fungal CFUs remained relatively constant in controls (Fig. 3b).  There was a large increase in CFUs after 3 days of incubation within the +BSM pots (Fig. 3b – note the log scale).  From 3 days of incubation on, only one species was observed from the +BSM treatment (data not shown). • To date, all but one of the seedlings that died have tested positive for Pythium (data not shown). • Materials and Methods • Potting mix consisted of 50% peat, 25% soil (Shano silt loam), 20% perlite and 5% compost (Eko, Lewiston, ID).  Soil was confirmed to contain Pythium. • Brassica juncea cv. ‘Pacific Gold’ seed meal (from Matt Morra, Univ. Idaho) was amended into potting soil at 3 g BSM /L (+BSM treatment) or not (control). • Control and +BSM potting mixes were wetted to 60% water holding capacity and allowed to incubate at room temperature in covered, plastic tubs. • At periodic intervals, spinach (cv. ‘Space’, germination rate 96%) was seeded into 2 L pots brought to 80% water holding capacity and placed in the greenhouse, with 3 replicates per treatment. • The experiment was repeated twice, with seeding/incubation times for the first experiment of 0, 7, 15, 28, 42 and 64 days, and for the second experiment of 0, 1, 3, 7, 14 and 28 days. For the second experiment, 28-day emergence and mortality are not reported here because we are still collecting the data. • Each pot received 10 seeds which were monitored daily for seedling emergence and mortality.  When no seedlings emerged, mortality for that pot was considered 100%. • In the 2nd experiment, at each potting day Pythium and total fungal populations (CFU) were counted after dilution plating onto 0.1-strength potato dextrose agar with rifampicin (75 μg/ml), benomyl (0.5 μg/ml) and difenconazole (1 μg/ml). • All emergent seedlings that died were surface-sterilized and plated to assay for the presence of Pythium. Potted spinach from first greenhouse experiment (peripheral pots) and second experiment (inset) Discussion When seeded immediately into potting mix that had just been amended with BSM, spinach was destroyed, probably due to the toxicity of AITC (Brown and Morra 1995). But seeding after only one day of incubation with BSM, emergence of spinach was improved over un-amended, Pythium-contaminated potting mix. And after 2 or 3 weeks of incubation with BSM, spinach emergence was nearly equivalent to the germination potential of the seed lot. The high emergence of spinach in BSM-amended potting mix that had incubated for some time, relative to emergence in the control mix, was an indication that the pathogenicity of the +BSM potting mix was reduced. The observation that viable Pythium colony forming units were reduced to below the level of detection with the dilution plating method we used supports the claim of reduced pathogenicity of the +BSM potting mix. The incredible increase in viable CFUs after 3 days of incubation in the +BSM potting mix was due to the growth of one taxon of fungus, probably Mortierella spp. No other species were noted on these plates. It may be that this non-pathogenic taxon was an effective competitor against Pythium, which may also have contributed to the reduced pathogenicity of the +BSM potting mix. Acknowledgements We thank Matt Morra for the seed meal; Logan Higgins, Kim Moles, Stephen Johnson, and Kalyani Muhunthan for help in lab and greenhouse and Jim Davis, Debbie Bikfasy and Kurt Schroeder for technical help. • Conclusions • Seeding immediately after incorporation of BSM into potting mix destroys the crop. • After about two weeks of incubation, BSM leads to high emergence and relatively low subsequent mortality of spinach. • Even though spinach mortality is low after fumigation with BSM, and viable Pythium is reduced, Pythium-induced damping-off still occurs. Literature cited Brown and Morra. 1995. J Agric Food Chem 43:3070-3074 Brown and Morra. 1997. Advances in Agron 61:167-231. Hoagland et al. 2008. Soil Biol Biochem 40:1689-1697. Matthiessen and Kirkegaard. 2006. Critical Rev in Plant Sci 25:235-265.