Biocontrol Activity of Nonpathogenic Strains of : Colonization on the Root Surface to Overcome Nutritional Competition.

is a soil-borne fungal pathogen that causes vascular wilts in a wide variety of crops. Certain nonpathogenic strains of are known to protect crops against pathogens. We assessed the biocontrol activities of nonpathogenic mutants of ff. spp. and generated by disruption of the gene, which encodes a Zn(II)2Cys6-type transcriptional regulator essential for their pathogenicity. Pre-inoculation of melon or tomato roots with strain Δ conidia markedly reduced disease incidence caused by the parental wild-type strain in a concentration-dependent manner of conidial suspensions of Δ strains. The biocontrol effect caused by the Δ pre-inoculation lasted for at least 7 days. Pre-inoculation of melon roots with the wild-type or Δ strain of f. sp. and nonpathogenic strain also led to biocontrol activity against f. sp. , indicating that the biocontrol activity of Δ strains is due to its nonpathogenic nature, not to the disfunction. Conidial germination and hyphal elongation of only the wild-type strain were inhibited on melon root surface pre-inoculated with conidia of strains nonpathogenic to melon plants. Expression of defense-related genes was not significantly induced in roots and aboveground parts of melon seedlings preinoculated with Δ conidia. Carbon source competition assay showed that nonpathogenic strains competed with the wild-type strain for a carbon source in soil. Strain Δ also competed with the oomycete pathogen for carbon source and protected melon plants from Our results suggest that the biocontrol activity of the nonpathogenic strains used in this study mainly depends on their extensive colonization of the root surface and outcompeting pathogens for nutrients.