High Amendment Lignin to Nitrogen Ratio Reduces Efficacy of Anaerobic Soil Disinfestation in Managing Strawberry Root Rot Pathogens

Cover crops, like winter wheat, are potential substrates for anaerobic/biological soil disinfestation (ASD) but lignin to nitrogen (N) ratio changes with growth stage and likely impacts ASD treatments due to decomposition rate effects. We evaluated wheat cover crop residue as an ASD amendment at three growth stages with varying lignin to N ratios on survival of Fusarium cugenangense, Globisporangium sylvaticum and Rhizoctonia fragariae. Treatments included 1) ASD treatment with vegetative stage residue, 1:1 lignin:N, 2) ASD treatment with early reproductive stage residue, 2:1 lignin:N, 3) ASD treatment with mature residue, 7:1 lignin:N, 4) ASD treatment with wheat bran, 1:1 lignin:N, and 5) a non-amended (non-ASD treatment) control. The experiment was conducted in a growth chamber at 25°C to 35°C. During incubation, high concentrations of volatile fatty acids (3.4 to 12 mmol VFA/kg soil at days 3 and 7) were detected in ASD treatments with low lignin to N ratio (1:1 or 2:1) cover crop residues, or wheat bran, which were higher than in ASD treatments with mature wheat residue and the non-amended control (< 1.5 mmol VFA/kg soil at days 3 and 7). ASD treatments with low lignin to N ratio cover crop residues significantly reduced F. cugenangense populations compared to all other treatments, with a 68 to 75% reduction compared to the non-amended control. Similar suppression of G. sylvaticum was observed with low lignin to N ratio cover crop residues compared to the non-amended control (> 96%), although wheat bran was also effective. For R. fragariae, survival was 55 and 73% lower in ASD treatments with vegetative stage residues or wheat bran compared to the non-amended control. Lower lignin to N ratio amendments enhanced VFA production and improved pathogen suppression; thus, using cover crops at earlier growth stages when lignin to N ratio is lower may optimize ASD outcomes.