Biological control of fusarium wilt of tomato by antagonistic bacterial strains

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Fusarium wilt, caused by Fusarium oxysporum f.sp. lycopersici, is an importantndisease on tomato worldwide, causing significant loss of yield. Fungicides do not givenenough protection against fiisarium wilt and thus control methods such as biologicalncontrol need to be developed. This thesis investigates the efficacy of bacterial strainsnIsolated from lucerne leaves and from the rhizosphere for biological control ofnfiisarium.Fungal pathogenicity tests showed that in the test system used, maximum expressionnof the wilt with disease severity of 3.2 (on a rating scale of 0 to 5), had occurred bynthe 5th week. Most wilt occurred at 28d C with an initial inoculum level of 106 conidianmL-1 (40 mL of conidial suspension per pot) poured evenly onto the surface of 750nmL of partially sterilized UC potting mix. Delaying the fungal inoculation timenreduced the severity of fiisarium wilt at the 5th week. Most disease occurred whennfungal inoculation was conducted at planting time or at 7 days after planting.nAlthough some seedlings were symptomless after inoculation with mycelial mats ofnF. oxysporum f.sp. lycopersici, their stems were colonized by mycelia of this fungusnwhen the stems were cut and plated on PDA. This stem plating method was used tonassess the fungal infection of tomato seedlings, as well as scoring symptoms withinnthe diseased plant. The type of fungal inoculum affected the level of infection ofntomato by F. oxysporum f.sp.lycopersici. The mycelial mat and oat seed inoculumnproduced most infected seedlings. Mycelial mats taken from cultures on carnationnleaf-piece agar (CLA, a minimal medium) or potato dextrose agar (PDA, a richnmedium), developed similar infection rates. Fungal infection increased over time upnto 21 days after planting.nOf 88 bacterial strains tested in vitro for their ability to inhibit F. oxysporum f.sp.nlycopersici growth on potato dextrose agar (PDA) plates, 30% showed strongninhibition. n