[Ultrastructural study of host-parasite relations: early phases of Venturia inaequalis installation on susceptible and resistance apple leaves].

The penetration mechanism study of Venturia inaequalis is undertaken on leaves of scab-susceptible and scab-resistant apple cultivars. The penetration through the cuticle occurs whatever the host phenotype. The resistance will be expressed when the infectious hyphae is in contact with the epidermal cell wall. The epidermal cell of susceptible variety does not show any modification. The resistant variety reaction is very fast, since twenty hours after penetration many wall appositions are setting, the cytoplasm strongly degenerates and the parasite hyphae are already necrotic. The contact between the host and the parasite is always very complex in the case of resistant host. The rapidity and the important cytological modifications of the host parasite interaction connect this reaction to an hypersensitive reaction.