Biological Control of Fusarium Wilt of Sweet Potato by Non-pathogenic Fusarium oxysorum

Abstract

Fungal hyphae were often found microscopically in stem vascular bundles of susceptible cultivar of sweet potato to Fusarium wilt, caused by Fusarium oxysporum f. sp. batatas. Some isolates of Fusarium oxysporum obtained from healthy tissues showed remarkable cross-protection against the disease, when they were previously inoculated to the sweet potato sprout before being planted in the infested soil. The isolates were not pathogenic not only to sweet potato but also to several species of major vegetable crops; such as cucumber, bottle-gourd, melon, Japanese radish, cabbage and tomato. Pre-inoculation with the non-pathogenic isolates of F. oxysporum was carried out by dipping fresh cut-end of sweet potato sprout into a bud-cell suspension of the isolates, prepared by shake-culture in potato-sucrose broth. Drenching the infested soil with the bud-cell suspension was insufficiently effective. Only the non-pathogenic isolates of F. oxysporum were effective among 12 pathogenic and non-pathogenic Fusaria belonging to 7 species tested, when they were preinoculated with the same method as described above. Pre-inoculation by dipping the cutend of sprout into a bud-cell suspension (10⁸cells/ml) was as effective in the naturally infested field as a chemical control by dipping them into 500 times of benomyl (50%W.P.) for 30 minutes. Smearing of a condenced suspension to the cut-end of sprouts was also as highly effective as dip treatment. The denser the bud-cell suspension was, and the longer the duration of dipping was, the greater protective effect. Pre-inoculation with the non-pathogenic F. oxysporum is considered to be applied for practical biological control of Fusarium wilt of sweet potato, because it is effective against the disease caused not only by soil-borne but also by the pathogen transmitted from tubers, and it does not require the introduction of any complicated operations.