Etiology and Control of Bulb Rot Disease of Onion (Allium cepae L.) in the Eastern Region of Ghana

Abstract

A study was carried out to assess farmers’ knowledge on prevalence, spread, control and economic importance of onion bulb rot disease in the Fanteakwa and Kwahu South Districts in the Eastern region of Ghana, determine the incidence and severity of the disease in the two districts and identify the causal organism of the disease and to develop a chemical control regime for management of the disease. Some local and exotic onion lines were screened for resistance to the disease. Eighty farmers were randomly selected from the experimental sites and interviewed using pretested semi- structured questionnaires. The questionnaires focused on the background of farmers, their knowledge and experiences on prevalence, spread, control and economic importance of the disease. Disease incidence was determined by counting the number of both healthy and diseased onion plants and disease severity was scored using a one to five disease assessment key on five onion farms in each operational area. The incidence and severity of the disease was higher in the Kwahu south district than the Fanteakwa district. Fusarium f. sp. cepae was consistently isolated from diseased samples brought from the field using morphological characteristics and PCR detection of the genomic DNA from the isolate. Four antagonistic agents comprising of two biological agents namely Trichoderma asperellium and Bacillus subtilis and two synthetic chemicals (Carbendazim and Mancozeb) were assayed on PDA against Fusarium oxysporum f. sp. cepae. The two synthetic fungicides resulted in 100% inhibition of mycelial growth of the fungus. Trichodema asperelium induced 76.5% inhibition while the bacillus subtilis resulted in 20% inhibition of mycelial growth after six days of incubation. Bawku Red produced the highest seedling emergence rate of 72.16% and the highest seedling survival rate of 65.70% following inoculation with the fungus in the screening obbays. emergence and survival rates. Texas Grano had the lowest seedling Mature plant screening was conducted on three cultivars (Bawku Red, Malavi and Red Creole). The least affected variety was Malavi with lower disease incidence (54.15%) severity (1.57) and also produced bigger bulbs compared to Bawku Red and Red Creole. The most affected was Bawku Red which also produced smaller bulbs as compared to the others. The controls were not affected with mean bulb weight of not less than 42 g for all the varieties. The efficacy of four antagonistic agents namely Carbendazim, Mancozeb, Trichoderma asperellum and Bacillus subtilis were evaluated against Fusarium oxysporium f. sp cepae. Malavi, a susceptible variety identified from the seedling screening was used as the experimental material in inoculated soil. Carbendazim was most effective fungicide. Bacillus subtilis was found to be least effective with disease incidence of 22.2% and severity rate of 1.2 recorded on onions treated with the organism. For management of the disease, Carbendazim or Mancozeb treatment at nursery, transplanting and spraying at four and eight weeks before harvest could be used alongside farm sanitation.