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).
We thank Howard Judelson for giving us plasmids pHAMT35G and pTH209, Leontine Colon for providing us with the potato cultivars, John van’t Klooster
for help with the fluorometric assay, Wim Blok for help with the figures and Pierre de Wit for critically reading the manuscript. This work was supported by the European Commission, project AIR2-CT94-1263. P. van West was supported by NWO-SLW project 805-22.461.