In a bid to enhance storability of highly perishable and seasonal onion bulbs in Nigeria, a study was conducted to control post harvest spoilage of onions using crude extracts of Jatropha curcas mainly from stem and leaves. Fungi associated with onion bulb rots were isolated on Potato Dextrose Agar medium and their pathogenicity was established by testing for their ability to induce rot in healthy onion bulbs. Ethanol extracts of leaves and stems of J. curcas were then screened for the potential to control a strain of Aspergillus niger v. Tieghem which had the highest frequency of occurrence on onion bulbs. Plate and broth assays were used at 0%, 10%, 20%. 30% and 40% concentrations of extract in Potato Dextrose Agar and Potato Dextrose Broth respectively. The stem extracts inhibited the growth of the fungus at all concentrations tested. A progressive inhibition with increasing concentration of the stem extract was observed on both plates and broth cultures. On the other hand, lower concentrations (10%, 20%, and 30 %) of leaf extract did not inhibit fungal growth on agar plates. Some level of inhibition was however observed at 40% concentration of leave extract. The phytochemical screening of the ethanolic extracts of Jatropha stem and leaves extracts showed that they contain high levels of saponin, tannins, phenolics, steroids and glucosides but contain little amounts of phylobatinin. The stem extracts had alkaloids in addition to other chemical constituents. It was concluded that the stem extract have the potential for use in controlling black mould of onion bulb.