Biocontrol of Fungi Associated with Onion (Allium cepa) Bulb Rot during Post Harvest

Post-harvest deterioration is a major problem of onions (Allium cepa) in Nigeria. The use of chemicals has been the main control measure. Chemicals are hazardous and environmentally unfriendly. There is therefore, a need for search for effective bio-pesticides as alternatives. A survey of fungal onions diseases was conducted in two markets in Mkpat Enin Local Government Area, Akwa Ibom State, Nigeria. Two hundred onions bulbs (100 from each market) were collected separately from the two markets. The collected bulbs were put into separate polythene bags before taken to the laboratory for macroscopic and microscopic examinations. Fungal isolates were obtained from naturally infected bulbs and their pathogenicity confirmed using Kochs’ postulate. In vivo and in vitro testing based on growth inhibition were also carried out to determine the efficacy of the aqueous plant extracts. Phytochemical analysis of alcohol extracts was conducted following standard procedures. Results of market surveys indicated disease incidences of 26% and 23% from Akpaden and Ukam markets, respectively. Three fungal species were isolated and identified as Botrytis allii, Fusarium oxysporum and Alternaria porri. Pathogenicity tests confirmed these isolates to be common agents of onion bulb rot. In vitro testing of Chromolaena odorata and Mangiferia indica leaf extracts at 10, 20 and 30% concentrations showed inhibitory effects of both extracts with increasing concentrations on the pathogens. C. odorata extracts indicated significantly higher inhibition, compared to M. indica. In vivo tests of the extracts also showed a significant growth inhibition of the pathogens on onion bulb rot, compared to the control. Phytochemical screening of the extracts indicated the presence of tannins, saponins, polyphenols and flavonoids. The antifungal activities of these extracts and their availability makes them potential control agents of fungal onion rot. C. odorata and M. indica plant extracts should therefore, be further explored as alternatives bio-control agents or onions fungal rot.