Storage rot fungi and seed-borne pathogens of onion

Surveys of fungi associated with postharvest deterioration of onion bulbs in four major markets in Kumasi Metropolis of the Ashanti Region of Ghana were conducted. Rotten onion bulbs obtained from the four markets (Abinchi, Anloga, Kwadaso and Central markets) were infected by five fungal species: Aspergillus niger, A. flavus, Penicillium sp., Rhizopus stolonifer and Fusarium oxysporum. Of these, A. niger and Penicillium sp. were the most frequently isolated fungi. A. flavus was the least encountered fungus. R. stolonifer and A. niger were the most pathogenic. Black mould, blue mould, soft rot, neck rot and basal plate rot were the major storage rots identified in a dry season survey in the markets. Incidences of these storage rots were higher in the wet season. Black mould enjoyed the highest incidence in all markets. Using the moist blotter method as recommended by the International Seed Testing Association (ISTA), seed-borne mycoflora of 37 samples of farmer-saved onion seeds from Bawku in the Upper East Region of Ghana were also examined. Nine different fungal species were identified and isolated from the seeds viz. A. niger, A. flavus, Penicillium sp., R. stolonifer, Fusarium oxysporum, Fusarium verticilloides, Fusarium poae, Alternaria porri and Botrytis sp. The most frequently encountered fungi were A. niger, R. stolonifer, A. flavus, Penicillium sp. and F. verticilloides with the occurrence of 33.3, 32.5, 25.8, 3.1 and 3.0%, respectively. The postharvest disease with the highest incidence in all the four markets surveyed was black mould. Some seed-borne fungi of onion were found to be associated with the onion bulb rots. Therefore, infected onion seeds could serve as inoculum source of the postharvest diseases of onion and contribute to the rot of bulbs during storage. In general, practices such as rotation, sanitation, optimum fertilization, preventive chemical applications, harvest timing and proper handling, harvesting, and storage are recommended for onion disease management.Keywords: Post-harvest, seed-borne, mycoflora, pathogenic, fungi