A transgenic Phytophthora infestans strain that constitutively produces and secretes β-glucuronidase (GUS) was used in detached leaf assays to quantify the levels of resistance to late blight in potato cultivars Surprise, Irene, Pimpernel, Alpha and Bintje. Four days after inoculation levels of GUS activity were determined in infected leaves. Significant differences between the various cultivars were observed. Discrimination between resistant and susceptible cultivars was possible based solely on levels of GUS activity. Regression analysis revealed a positive correlation between in planta GUS levels and field resistance expressed as Area under Disease Progress Curve (ADPC).