Role of Organic Amendment Composition and Soil Texture in Modulating Volatile Fatty Acids, Fe/Mn Reduction, and Fusarium oxysporum Suppression During Anaerobic Soil Disinfestation in Neutral to Alkaline Soils.

Less hazardous alternatives to soil fumigants for suppressing soilborne pathogens such as pathogenic isolates within the Fusarium oxysporum (Fo) species complex that cause black root rot of strawberry are urgently needed. A promising alternative is anaerobic soil disinfestation (ASD), in which anaerobic fermentation is induced in soil, leading to production of pathogen-suppressing reduced metal cations (Fe²⁺, Mn²⁺) and volatile fatty acids (VFAs) before planting. However, little is known about how interactions between amendment composition, soil texture, and neutral to alkaline soil pH influences Fo suppression via soil biogeochemistry. Suppression of Fo was investigated in soil-based ASD incubation mesocosm trials where Fo-inoculated soils with initial neutral pH (sand, sandy loam, and silty clay) were amended with lime (0.1% CaCO₃) or unamended, and amended (1% w/w) with five amendment mixtures of soybean protein isolate (SPI) and dried molasses (DM) (100/0%, 75/25%, 50/50%, 25/75% or 0/100%) for a range of protein: carbohydrate ratios (0.1:1 to 32:1), to initiate ASD treatments. Post-ASD, soil chemical properties were measured, including VFA and Fe²⁺/Mn²⁺ concentrations, and Fo inoculum viability was assessed. Total soil VFA and Fe²⁺/Mn²⁺ concentrations were influenced by ASD amendment composition and soil texture, with the highest VFA concentrations resulting from high protein amendments and the highest Mn²⁺ concentrations observed in sandy loam and silty clay soils. Fo viability was reduced in all amended treatments, but finer soil textures were associated with lower Fo suppression. Our results highlight the importance of soil texture and amendment composition in influencing ASD effectiveness in neutral to alkaline soils.