Reduction in weed seedling emergence by pathogens following the incorporation of green crop residue

Because tillage promotes the germination of many weed species and freshly killed plant material favours the growth of microbial pathogens, we hypothesised that the incorporation of green crop residue should temporarily reduce weed seedling emergence relative to unamended soil. Soil with field-incorporated green crop residue was compared with non-amended soil in glasshouse experiments by sowing several weed species at different times after incorporation. Species included Abutilon theophrasti, Chenopodium album, Amaranthus powellii, Setaria faberi, Echinochloa crus-galli and, in one year, lettuce and red clover. Soils with green crop residue reduced seedling emergence for 0–4 days after incorporation by an average of 30%. Comparison of emergence in nonsterilised soil with that in sterilised soil, with and without fresh crop residue, indicated that a biological agent caused the depressed emergence. In the third year of the study, the fungi Fusarium oxysporum and F. chlamydosporum were isolated from seeds exposed to soil amended with green crop residues, and their pathogenicity to seeds and seedlings was confirmed in bioassays. This study indicated that incorporation of fresh crop residue reduces the first flush of weed seedlings following tillage and that this depression in emergence is probably caused by pathogen attack on seeds and seedlings before emergence.