First evidence of the breakdown in Belgium of the scab resistance conferred by the Vf gene.

Scab is the most important fungal disease of apple throughout the world. The main breeding programmes are focused on scab resistance conferred by monogenic sources of resistance, specifically the Vf gene originated from Malus floribunda 821. Since 1993, evidence of the breakdown of this resistance mechanism appeared in different countries like United Kingdom, Germany, The Netherlands and France. In 1997, a first evidence of the Vf gene breakdown in Belgium was observed on the cultivar 'Judeline' planted in an untreated orchard at Aubel where a cider making company, with our collaboration, search for cultivars adapted for juice and cider processing. Therefore, a glasshouse experiment was planned in order to characterize which scab race was present. In 1999, the artificial inoculation of some cultivars in controlled conditions showed that the scab race collected the year before at Aubel presented the same characteristics than race 7 described by other works. At the same time, an orchard was established by our Department as planned in an European FAIR project named 'Durable Apple Resistance in Europe' (D.A.R.E) with the aim to study the resistance durability of some partial scab resistant apple cultivars inside an European orchard network. Sixteen repetitions of 20 different cultivars are planted and are originating from 8 participating countries. Five apple clones with specific host resistance towards 4 specific scab races are also present. Incidence and severity assessments of the disease are made 3 times a year on the whole tree according to two international standardized scales. For the first time in 2000, a specific scab race was observed on Malus floribunda 821 which caused very serious injuries. Those results confirm that new breeding strategies must be developed for reducing the risk of scab resistance breakdown in the future.