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 years in apple breeding programs. Since 1984, scab symptoms have been observed in the field at Ahrensburg, Germany, on seedlings of apple cv. Prima that have been selected as resistant in the greenhouse. In 1988, small scab lesions were found on some Vf selections in the same orchard. The inoculum from Ahrensburg was compared with the inoculum currently used at Angers, France, for selecting apple seedlings for resistance to V. inaequalis. All Vf-gene cvs. or selections tested were susceptible to the Ahrensburg inoculum, whereas M. floribunda 821 itself and the ornamental crabapple Evereste were resistant. The progeny from a cross between a resistant (Vf) and a susceptible cv. segregate into the 5 expected classes to Angers inoculum, but were completely susceptible to Ahrensburg inoculum. These results indicate the urgency of diversifing the sources of resistance to V. inaequalis in new breeding strategies. The distinction should be made between the resistance of M. floribunda 821, that is resistant to Ahrensburg inoculum, and that of the named cv. and selections, that are susceptible. The new race of V. inaequalis is named race 6.