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.