Relations between capsidiol concentration, speed of fungal invasion and level of induced resistance in cultivars of pepper (Capsicum annuum) susceptible or resistant to Phytophthora capsici

Phytophthora capsici invaded the decapitated stem of susceptible cultivars of Capsicum annuum Yolo Wonder and Clairon at an approximately constant rate over 9 days following inoculation, while it became progressively inhibited in the respective isogenic resistant cultivars Phyo 636 and Fidelio. Capsidiol concentration at the advancing edge of the mycelium, although somewhat higher in the resistant cultivars, never exceeded the ED 80 for in vitro activity; its kinetics were similar in the 4 cultivars, with a maximum around the 4th day and a marked decrease afterwards. In the resistant cultivars only, infection progressively induced a persisting state of resistance inside the tissues, demonstrated by a subsequent inoculation. Upon infection, induced tissues accumulated rather less capsidiol than non preinoculated ones. Degree of fungal growth inhibition and level of induced resistance were closely correlated, but bore no overall relation with capsidiol concentration at the front of the mycelium. These results suggest that, although active on the fungus inside the host tissues, capsidiol is not the main factor responsible for cessation of fungal growth in the resistant plants. Another, as yet unknown defence mechanism, rendering the tissues inhibitory towards further invasion must be implicated in the cultivar-specific resistance of pepper.