ABILITY OF GRAPEVINE ROOTSTOCKS VARIETIES TO TRANSMIT FLAVESCENCE-DOREE - STUDY OF THE CASE OF 3309-C AND FERCAL

Most grapevine rootstock varieties are potential symptomless carriers of Flavescence doree (FD). However some infected plants of 3309 C and Fercal rootstocks can be recognized as they exhibit late spring growth and incomplete ripening of some canes. Once infected, these plants re main as potential sources of infection throughout their lifetime. Ther efore, contrary to V vinifera behaviour, treating fields with insectic ides against the leafhopper vector to prevent inoculations of the dise ase is not efficient in reducing the infectivity of the plants already infected. Only 6-80% of cuttings obtained from infected plants transm it the disease. Canes from the same vine are unevenly infected and the infection level along the canes is also uneven. The high degree of FD transmission in 3309 C is associated with a high graft mortality, the 'practical mortality' (graft death + diseased) sometimes reaching 82% . New inoculations can occur in fields containing 3309 mother plants, even though insecticide treatments are mandatory. This stresses the ne cessity of a geographical isolation of mother-plant stands.