Colonization of cucumber plants by the biocontrol fungus Clonostachys rosea f. catenulata

Clonostachys rosea f. catenulata (Gliocladium catenulatum) strain J1446 (formulated as Prestop WP) suppressed Fusarium root and stem rot caused by Fusarium oxysporum f. sp. radicis-cucumerinum (Forc) on cucumber plants grown hydroponically in rockwool medium. Sixty days following application at seeding, the biocontrol agent had proliferated through the rockwool blocks and was present on cucumber roots and the crown region of the stem at populations >1 × 10⁵ CFU/g fresh weight. Scanning electron micrographs showed that C. rosea had rapidly colonized the root surface and was associated with root hairs and epidermal cell junctions. Following transformation of the fungus with Agrobacterium tumefaciens strain AGL-1 containing the hygromycin resistance (hph) and β-glucuronidase (uidA) genes, blue-stained mycelia could be seen growing on the surface and within epidermal and cortical cells of roots, stems and shoots 3 weeks after treatment. Quantification of GUS activity by fluorometric assays showed that fungal biomass was highest in the roots and crown area, while the extent of colonization of upper stems and true leaves was variable. Higher population levels resulted following application to rockwool blocks compared to seed treatment. Application of C. rosea preceding inoculation with Forc significantly reduced pathogen populations on roots compared to plants inoculated with Forc alone. Colonization of infection sites in the root zone reduced pathogen development and disease incidence. Densities of the biocontrol agent appeared to increase in the presence of the pathogen.