Effects of apple cultivar susceptibility to Venturia inaequalis on scab epidemics in apple orchards

The effects of cultivar susceptibility on the development of scab caused by Venturia inaequalis were observed in two experimental orchards where no fungicide protection was used against scab over a 5-year period. The weather during a 4-year period was conducive to apple scab development, and the results obtained demonstrate the advantages of using low-susceptibility cultivars to limit the development of apple scab epidemics on leaves as well as on fruits. Whereas 95.2–100% of the fruits of susceptible cultivars had apple scab at harvest, low-susceptibility cultivars were relatively less scabbed with only 3.1–46.5% of scabbed fruits. During years with low scab stress, it was very interesting to observe the almost total absence of the disease on the low-susceptibility cultivar, Reine des Reinettes, whereas on the susceptible cultivar, Gala, 40% of leaves and 22% of fruits were scabbed at harvest. These results confirm the interest of partial resistance for scab management in orchards. It is possible to take the level of susceptibility of cultivars into consideration within the framework of integrated fungicide protection against apple scab.