Role of native soil biology in Brassicaceous seed meal-induced weed suppression

Biologically based weed control strategies are needed in organic and low-input systems. One promising practice is the application of Brassicaceous seed meal (BSM) residue, a byproduct of biodiesel production. When applied as a soil amendment, BSM residue has exhibited potential bioherbicide activity. In this study, tree fruit orchard soils were treated with various BSMs and the impact of Pythium on weed suppression was examined in field and greenhouse studies. Although weed control obtained in response to Brassicaceous residue amendments has been repeatedly attributed solely to release of allelopathic phytochemicals, multiple lines of evidence acquired in these studies indicate the involvement of a microbiological component. Reduced weed emergence and increased weed seedling mortality were not related to BSM glucosinolate content but were correlated with significant increases in resident populations of Pythium spp. in three different orchard soils. Seed meal of Brassica juncea did not amplify resident Pythium populations and did not suppress weed emergence. Application of Glycine max SM did stimulate Pythium spp. populations and likewise suppressed weed emergence. Application of a mefenoxam drench to Pythium-enriched soil significantly reduced weed suppression. These studies indicate that a microbial mechanism is involved in SM-induced weed suppression and that selective enhancement of resident pathogenic Pythium spp. can be utilized for the purpose of weed control.