A MICROSCOPIC STUDY OF THE DIFFERENT CLASSES OF SYMPTOMS CODED BY THE VF GENE IN APPLE FOR RESISTANCE TO SCAB (VENTURIA-INAEQUALIS)

The different symptoms of resistance coded by the Vf gene were studied on glasshouse-grown apple seedlings after inoculation with a conidial suspension of Venturia inaequalis. Scanning-electron microscopy and histological observations defined precisely three different classes of symptoms. Class 1 corresponds to a hypersensitive reaction. Class 2 is characterized by considerable tissue modifications but no sporulation whatsoever occurs; this reaction defines resistant hybrids. In class 3 the fungus remains active and can sometimes sporulate abundantly; this class was divided into two subgroups, one comprising all the hybrids of weak resistance, the other the hybrids of weak susceptibility.