Abstract A vegetable oil was used as the major constituent to develop an invert emulsion for use with mycoherbicidal agents. The oil phase (CPWA) contained soybean oil, a paraffinic spray oil, paraffin wax, and the emulsifying agents Arlacel 780 and Arlacel 989. When mixed with water (50:50), a stable, relatively low viscosity, water-in-oil emulsion was formed, which was sprayed onto glass plates and subjected to various environmental conditions to determine the water-retaining properties. This highly stable formulation was capable of retaining 70% of the original water content after 20 h of incubation at 22°C and 70% relative humidity. Even at 25°C and 40% relative humidity, 40% of the original water content remained in the sprayed emulsion droplets after the same incubation time. When spores of the fungus Ascochyta pteridis, a pathogen of bracken, were incorporated into the formulation and sprayed onto glass plates, 85% of spores germinated within 20 h and mycelial growth was prolific at 22°C and 70% relative humidity. The efficacy of this formulation on pot-grown bracken was relatively low, with 27.9% necrosis observed after 28 days, due to the particularly high resistance to fungal attack of this bracken. However, the potential of this formulation has been shown and it may be useful for other weeds and their respective mycoherbicide agents.