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

Cover crops, like winter wheat, are potential substrates for anaerobic/biological soil disinfestation (ASD), but the lignin-to-nitrogen (N) ratio changes with growth stage and likely impacts ASD treatments owing 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 (i) ASD treatment with vegetative-stage residue, 1:1 lignin/N; (ii) ASD treatment with early reproductive-stage residue, 2:1 lignin/N; (iii) ASD treatment with mature residue, 7:1 lignin/N; (iv) ASD treatment with wheat bran, 1:1 lignin/N; and (v) a nonamended (non-ASD treatment) control. The experiment was conducted in a growth chamber at 25 to 35°C. During incubation, high concentrations of volatile fatty acids (VFAs) (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 nonamended 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 with all other treatments, with a 68 to 75% reduction compared with the nonamended control. Similar suppression of G. sylvaticum was observed with low lignin-to-N ratio cover crop residues compared with the nonamended 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 with the nonamended control. Lower lignin-to-N ratio amendments enhanced VFA production and improved pathogen suppression; thus, using cover crops at earlier growth stages when the lignin-to-N ratio is lower may optimize ASD outcomes.