Host‐Parasite Relationship in a Susceptible and a Resistant Rose Cultivar Inoculated with Sphaerotheca pannosa: I. Fungal Growth, Mechanical Barriers and Hypersensitive Reaction

Conidial germination and development of Sphaerotheca pannosa were studied on a susceptible and a resistant cultivar (rose Cardinal and rose Queen Elisabeth, respectively) in relation to the morphology of the leaf surface which was examined by scanning and transmission electron microscopy. Conidial germination was completed within 24 h in both cultivars and seemed to be higher in the resistant cultivar; fungal development, however, was stopped when the haustoria containing cells became necrotic. The only morphological difference between the leaves of the two cultivars is a thinner cuticle in rose Cardinal. Wax deposits on the leaf surface are similar; a layer of substances reacting to rhutenium red, most likely glycoproteins or protein‐polysaccharide complexes, also are similar.