A NEW RACE OF VENTURIA-INAEQUALIS VIRULENT TO APPLES WITH RESISTANCE DUE TO THE VF GENE

The gene Vf from Malus floribunda 821 for resistance to scab (Venturia inaequalis), has been used successfully for 50 yr in apple breeding p rograms. Since 1984, scab symptoms have been observed in the field at Ahrensburg, Germany, on seedlings of apple cv. Prima that have been se lected as resistant in the greenhouse. In 1988, small scab lesions wer e found on some Vf selections in the same orchard. The inoculum from A hrensburg was compared with the inoculum currently used at Angers, Fra nce, for selecting apple seedlings for resistance to V. inaequalis. Al l Vf gene cultivars or selections tested were susceptible to the Ahren sburg inoculum, whereas M. floribunda 821 itself and the ornamental cr abapple Evereste were resistant. The progeny from a cross between a re sistant (Vf) and a susceptible cultivar segregate into the rive expect ed classes to Angers inoculum, but were completely susceptible to Ahre nsburg inoculum. These results indicate the urgency of diversifying th e sources of resistance to V. inaequalis in new breeding strategies. T he distinction should be made between the resistance of M. floribunda 821, which is resistant to Ahrensburg inoculum, and that of the named cultivars and selections, which are susceptible. The new race of V. in aequalis is named race 6.