Host-Parasite Relationship in a Susceptible and a Resistant Rose Cultivar Inoculated with Sphaerotheca pannosa.

Young leaves of two rose cultivars, one susceptible (rose Cardinal) and the other moderately resistant (rose Q. Elizabeth) to mildew infection, were either inoculated with Sphaerotheca pannosa or wounded with glass or metal needles. At different times after these treatments the presence of callose, lignin and phenolics in the infected or wounded cells was revealed by histochemical reactions. After either treatment no difference was found between the cultivars in the amount or earliness of deposition of callose or lignin, while phenolics were present in far larger amounts in the resistant cultivar. It is therefore suggested that one of the factors of rose resistance to powdery mildew might be the capacity to synthesize large amounts of phenolic compounds, also in view of the fact that the vacuoles of healthy epidermal cells are strongly electron opaque in rose Q. Elizabeth, while they are much more transparent in rose Cardinal.