Organic acids in condensed distiller's solubles: Toxicity to soilborne plant pathogens and role in disease suppression

Condensed distiller's solubles (CDS), a coproduct of ethanol production from corn (Zea mays), has shown disease suppressing effects when added to pathogen-infested soils and peat-based substrate prior to planting. In this study, CDS amendment (1% and 3% w/w) to a sandy-loam soil displayed a low level of toxicity to microsclerotia of Verticillium dahliae and reduced their germination by 46% to 63% in 1 week in the laboratory soil microcosm tests. The analyses of CDS for the presence of any toxic substances revealed that it contains moderate levels (~144 mmol/L) of volatile (acetic and formic) and nonvolatile (glycolic) organic acids, and some of these are known toxicants. Glycolic, acetic, and formic acids contents were 77.1%, 17.7%, and 3.9%, respectively. Minor concentrations (0.2- 0.7 mmol/L) of propionic, n-butyric, isobutyric, n-caproic, and n-valeric acids were also found in CDS. In solution assays, the viability of V. dahliae microsclerotia treated for 24 h in 1%, 2%, 5%, and 10% (v/v) CDS (pH 3.6-4.5) or a mixture of organic acids (pH 3.2-5.0) with the same percent composition as the CDS was reduced by 2%, 7%, 22%, and 48% or 6%, 32%, 53%, and 69%, respectively. A mixture of organic acids with the same volumetric ratios as 2% and 4% CDS completely inhibited the growth of Pythium ultimum after treatment of culture plugs for 24 h in solution assays. In growth room bioassays, addition of a mixture of organic as in 1% and 2% CDS (v/w) to a P. ultimum infested muck soil reduced damping-off severity by 45%-52% and increased the percentage of healthy seedlings by 164%-180% over the control. A pre-planting amendment of glycolic acid (0.075% and 0.15% w/w) to an infested muck soil significantly increased the percentage of healthy cucumber (Cucumis sativus) seedlings by 107% and 122%, respectively, and decreased the damping-off severity by 33% and 40%, respectively, over the control. This study suggests that organic acids from CDS have a role in disease suppression in sandy-loam and muck soils.