In greenhouse experiments, unrefined corn oil was tested as an adjuvant with the mycoherbicidal fungus, Colletotrichum truncatum. A1:1 (V/V) fungus/corn oil tank mixture extended by at least 70 h the period of time in which the fungus remains infective between inoculation and the onset of dew. This formulation also reduced the dew period requirements for optimal weed pathogenesis and mortality from 12 h to 2 h, and reduced the required spray volume by 100-fold, from 500 L/ha to 5 L/ha. These results suggest that formulating C. truncatum in unrefined corn oil greatly improves the bioherbicidal potential of this pathogen.