Monitoring and occurrence of new races of Venturia inaequalis on apple in Belgium.

Since 1999, inside the framework of the European research project--FAIR5 CT97--3898 'Durable Apple Resistance in Europe'--a monitoring of scab infection on Leaves on 25 apple cuttivars (cvs)--including 10 differential hosts for scab races and European old cvs with partial scab resistance--was carried on trees in an unsprayed scab trap orchard planted at Gembloux, Belgium. The occurrence of scab races was determined by the use of a global assessment scale from 1 (no symptom) to 9 (maximum susceptibility) taking into account of both incidence and severity of the disease. Since 2004, scab was also scored according to a second evaluation scale based on qualitative symptoms. On the basis of the occurrence of symptoms on the various differential cvs, seven described physiological races of V. inaequalis were progressively identified in the orchard. During this period, important differences in incidence and severity of scab were observed on the different cvs. The susceptible cvs 'Golden Delicious', 'Gala' and 'Fiesta' were the most infected. On 13 polygenic varieties, sporulation was more or less important according to the disease pressure and their degree of susceptibility. The presence of race 7 on trees of Malus floribunda 821 has already been reported since 2000 year. Races 5, 6, 7 and 13 sporulated rather abundantly on their specific hosts according to the season's conditions although races 2 and 4 incited limited infection on their differential hosts and only under high disease pressure. First occurrence of race 3 in Belgium was observed in 2008.