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Biosolarization is an alternative to chemical fumigation that couples solarization and organic amendments to control diseases by increasing temperature and microbial activity. A field study was conducted to evaluate the efficacy of biosolarization, using almond processing residues from two varieties, to control propagules of Fusarium oxysporum f. sp. lactucae, the causal agent of Fusarium wilt of lettuce. Peak soil temperatures of solarized and biosolarized soils were 6–15 °C higher than control soils, and biosolarized soils experienced 1 to 3 °C higher temperatures on average and 10 to 27°C-day higher cumulative temperatures than soil solarized without amendment. Residues were high in endogenous organic acids, between 144 and 298 mM combined acetic, lactic, succinic, and butyric acids. After eight days of treatment, solarized soils only reduced the population of Fusarium oxysporum f. sp. lactucae by 9% compared to the average non-treated non-amended control plots; whereas soils solarized and amended with residues from pollinator varieties of almonds showed a greater pathogen reduction of 63%. In this instance, Fusarium wilt had no effect on plant health due to low pressure and moderate innate plant resistance. Soils amended with nonpareil residues had similar lettuce biomass and health scores to untreated plots, but lower health scores than the solarized treatments. This may be due to residual organic acids detected in biosolarized plots 3–4 weeks after treatments (1–57 mM), which significantly correlated to poor lettuce health score. Roughly, one year after soil treatment, solarized plots continued to out-perform pollinator-residue amended plots. Germination assays indicate biosolarization outcomes may be improved with increased aerobic remediation time. |
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