Ripe rot caused by Colletotrichum acutatum (CA) has been the most serious preharvest disease of grape berries/clusters. Furthermore, the pathogen is highly resistant to benomyl and iminoctadine-triacetate. A rapid assay for screening ripe rot (CA) resistance of breeding materials in table grapes was developed in which the harvested berries are washed with water and surface-sterilized in ethanol. The berries were then inoculated with conidial suspensions (ca. 3.0 × 105 spores/mL per ten berries) in a polyethylene bag, and incubated at 25°C in the dark for 2 weeks. The percentage of diseased berries increased as the ripening of berries proceeded. A period of 70 to 80 days after full bloom was suitable for berry sampling and subsequent CA inoculation to evaluate varietal differences in the ripe rot resistance of grapevine.