Onion is an important vegetable crop in Nigeria grown in both wet and dry seasons though it fetches higher profit when cultivated in the dry season. The crop yield is limited by some production constraints, notable of which is the „Danzazzalau‟ disease in Kebbi State. Fungi isolated from infected pseudostem and roots of onion were Fusarium sp. and Thielavia sp. Pathogenicity test was conducted using these fungi and Fusarium sp. proved to be pathogenic on 6-week old onion seedlings. Fusarium sp. was identified as a Fusarium species belonging to biological group clade V. by Centre for Agriculture and Bioscience International (CABI), Egham, Surrey, United Kingdom. Trials were carried out in the screenhouse and field during 2015 dry season at Samaru, Zaria to study the effects of water regimes, manure rates and mulch treatments for the management of „Danzazzalau‟ disease of onion crop. The treatments consisted of three water regimes (3, 10 and 21-day), three manure rates (0 Kg/plot, 40 Kg/plot, and 80 Kg/plot) and three mulch types (polyethylene sheet, rice straw and no mulch) laid out in a Split Plot Design (SPD) with three replications. Water regime was the main plot while manure rates and mulch types were the sub-plots. Incidence and severity of the disease and its effect on onion growth and yield were determined. Lowest disease incidence (36.02 %) and severity (32.93 %) were obtained at the 21-day water regime. The 21-day water regime and rice straw mulch at 1.2 mt/ha produced highest bulb yield of 4.67 t/ha and 4.80 t/ha respectively which were significantly (P≤ 0.05) higher than the other treatments. The efficacy of aqueous and methanol crude extracts obtained from Jatropha curcas, Moringa oleifera and Eucalyptus globulus leaves at 5, 7.5 and 10 g/ml on growth and development of Fusarium clade V. was tested in vitro. Aqueous extract of Moringa at 10 g/ml (5 % w/v) when used to amend culture media led to decrease in number of conidi