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Perilla frutescens (L.) Britton, an annual plant in the mint family, is considered invasive in the United States where it excludes native vegetation in natural areas and can cause respiratory illness in livestock. An effective biological control agent would improve perilla management, but none have been developed or proposed. This study investigated the biocontrol potential of a fungal pathogen, Colletotrichum shisoi, recently found infecting perilla in North America. The growth rates and morphology of seven Colletotrichum shisoi isolates were recorded. A virulence assay was performed on two perilla accessions, and the number and size of foliar lesions produced were used to select a single isolate, Colletotrichum shisoi isolate 21-072, exhibiting consistently high virulence for further evaluation of biocontrol efficacy and host specificity. Treatment with 0.5 mL of 1 × 107 spores/mL H2O caused a 52% reduction in shoot height and 84% reduction in root length of invasive perilla seedlings compared to a control treatment (P < 0.001). Seedlings treated with 1 × 108 spores/mL had a 100% mortality rate. Formulation of inoculum in a 1:1 water:oil emulsion reduced dew period requirements for causing >50% plant mortality from 48 to 24 h, but oil emulsions exhibited a mild phytotoxic effect on seedlings. Four additional perilla accessions were susceptible to C. shisoi, and none of 34 other species included in host range experiments developed disease symptoms following inoculation. These results suggest C. shisoi 21-072 could be an effective antagonist of perilla and likely poses a low risk of non-target impacts. [ABSTRACT FROM AUTHOR]