Control of soilborne disease by foliar spray of plant activators and analysis of its mechanisms

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Tomato wilt, caused by the soilborne fungus Fusarium oxysporum f.sp.lycopersici, is effectively controlled by a foliar spray of validamycin A (VMA) or validoxylamine A (VAA)(【greater than or equal】10 μg/ml) ; however, neither VMA nor VAA is antifungal in vitro. In pot tests, the effect of a foliar application of 100 μg/ml VMA or VAA lasted for 64 days. Plants sprayed with VMA or VAA accumulated salicylic acid, and had elevated expression of the systemic acquired resistance (SAR) marker genes P4(PR-1), Tag (PR-2), and NP24(PR-5). Foliar spray of VMA also controlled late blight and powdery mildew of tomato. The disease control by VMA and VAA lasted up to 64 days after treatment, was broad spectrum and induced the expression of PR genes, all essential indicators of SAR, suggesting that VMA and VAA are plant activators. The foliar application of plant activators is a novel control method for soilborne diseases and may provide an economically feasible alternative to soil fumigants such as methyl bromide.