Pseudoperonospora cubensis (Berk. et Curt.) Rost. the causal agent of downy mildew in cucurbits, produced large (10-15 mm) sporulating lesions in the susceptible muskmelon (Cucumis melo L.) cultivar Hemed (He) but small (1-2 mm), chlorotic, water-soaked lesions with negligible sporulation in the resistant inbred line PI 124111F (PI). The resistance of the inbred line was found to be temperature-dependent: it was maximal at 21-25 °C and minimal at 12-15 °C. At 12 °C large lesions with conspicuous sporulation developed in both the susceptible and in the resistant plants. Autofluorescence, callose and lignin-like deposits were observed in infected PI but not in He incubated at 21 °C. These structural responses were not observed in either PI or He incubated at 12 °C. Peroxidase activity and autofluorescence were higher in the resistant inbreds PI and F10(near isogenic line) as compared to He at 21 °C but not at 12 °C. We suggest that the resistance genes Pc-1 and Pc-2, which confer resistance to downy mildew in PI 124111F, are specifically inactivated at the lower temperature.