HISTOCHEMICAL-STUDIES ON THE INTERACTION BETWEEN 3 SPECIES OF GRAPEVINE, VITIS-VINIFERA, V-RUPESTRIS AND V-ROTUNDIFOLIA AND THE DOWNY MILDEW FUNGUS, PLASMOPARA-VITICOLA

Leaves of Vitis vinifera cv. Grenache (susceptible), of V. rupestris cv. du Lot (intermediate resistant), and of V. rotundifolia cv. Carlos (resistant) were inoculated with Plasmopara viticola and examined microscopically after staining with a series of reagents. These three species showed, respectively, necrotic spots without sporulation (resistant), diffuse necrosis with limited sporulation (intermediate resistant), and no visible necrosis with heavy sporulation of the parasite (susceptible). In the resistant species, flavonoid compounds were detected in the stomatal cells and the cells around the stomata 2 days after inoculation. In the intermediate resistant species, resveratrol and peroxidase activity were detected 5 days after inoculation, flavonoid compounds were present at 8 days, and lignin was formed in tissue surrounding the necrosis at 15 days. In the susceptible species, only a small amount of yellow autofluorescence was observed in the stomatal cells 8 days after inoculation. These data suggest that the rapidity of flavonoid formation plays an important role in the resistance of V. rotundifolia to P. viticola. The formation of resveratrol, flavonoids and lignin in the intermediate resistant species probably restricts the development of this pathogen.