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

Abstract

<i>Fusarium oxysporum</i> is a soil-borne fungal pathogen that causes vascular wilts in a wide variety of crops. Certain nonpathogenic strains of <i>F. oxysporum</i> are known to protect crops against <i>F. oxysporum</i> pathogens. We assessed the biocontrol activities of nonpathogenic mutants of <i>F. oxysporum</i> ff. spp. <i>melonis</i> and <i>lycopersici</i> generated by disruption of the <i>FOW2</i> gene, which encodes a Zn(II)2Cys6-type transcriptional regulator essential for their pathogenicity. Pre-inoculation of melon or tomato roots with strain Δ<i>FOW2</i> conidia markedly reduced disease incidence caused by the parental wild-type strain in a concentration-dependent manner of conidial suspensions of Δ<i>FOW2</i> strains. The biocontrol effect caused by the Δ<i>FOW2</i> pre-inoculation lasted for at least 7 days. Pre-inoculation of melon roots with the wild-type or Δ<i>FOW2</i> strain of <i>F. oxysporum</i> f. sp. <i>lycopersici</i> and nonpathogenic <i>F. oxysporum</i> strain also led to biocontrol activity against <i>F. oxysporum</i> f. sp. <i>melonis</i>, indicating that the biocontrol activity of Δ<i>FOW2</i> strains is due to its nonpathogenic nature, not to the <i>FOW2</i> 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<i>FOW2</i> conidia. Carbon source competition assay showed that nonpathogenic strains competed with the wild-type strain for a carbon source in soil. Strain Δ<i>FOW2</i> also competed with the oomycete pathogen <i>Pythium aphanidermatum</i> for carbon source and protected melon plants from <i>P. aphanidermatum.</i> Our results suggest that the biocontrol activity of the nonpathogenic <i>F. oxysporum</i> strains used in this study mainly depends on their extensive colonization of the root surface and outcompeting pathogens for nutrients.