Penetration and colonization of cucumber roots by pathogenic and nonpathogenic Fusarium oxysporum

In previous studies, a nonpathogenic strain of Fusarium oxysporum (C14) was shown to induce protection in cucumber to F. oxysporum f. sp. cucumerinum. Strain C14 induced protection whether roots were wounded, or intact, when inoculated with the nonpathogen, whereas another strain (C5) required wounds. Penetration of cucumber root and colonization of root tissue by the two biocontrol agents (strain C5 and C14) and F. oxysporum f. sp. cucumerinum, were further investigated using scanning electron, light and epifluorescence microscopy. In all the trials, no hyphal growth was detected inside, or outside, the noninoculated control cucumber roots. Strain C5 penetrated indirectly through the wounds but rarely through intact epidermis and hyphae were only observed in root tissues exposed to injury. Strain C14, which previously was shown to induce soil suppressiveness to the wilt pathogen significantly higher than strain C5, was similar to F. oxysporum f. sp. cucumerinum in the formation of penetration-like structures on intact cucumber roots. However, hyphae of strain C14 were found at discrete locations through the entire depth of the cortex but without extending towards the vascular bundles. Hyphae of F. oxysporum f. sp. cucumerinum were found, exclusively, deep within the cortex near the stele and usually completely surrounded the xylem vessels. From events presented in this and previous studies, we suggest that penetration and colonization of living intact root tissues by strains of nonpathogenic Fusarium spp. are necessary for efficient biological control of Fusarium wilt pathogens.