Résumé
To investigate the role of environmental conditions on the selection of virulent Potato virus Y (PVY) isolates subject to pressure from the recessive resistance gene va in tobacco, a field survey was performed in Brazil where va-derived genotypes have been recently introduced and now represent less than one-third of cultivated tobacco genotypes. A serological analysis of 397 leaves collected from different Brazilian tobacco-growing areas and mainly from plants with symptoms indicated that 52-4% of samples were infected by at least one of the viral species tested. PVY was present in 72.1% of infected samples. The probability of a plant being infected with PVY was reduced in va hosts. However, the biological characterization of PVY isolates on indicator hosts showed that 20 of the 29 tested isolates were able to overcome the alleles of the va gene. Moreover, the observed biological diversity of isolates was higher in susceptible tobacco genotypes than in va-resistant ones. Comparison of these data with the PVY diversity in French tobacco fields shows that the use of va-derived genotypes in two environments with contrasting climatic conditions, local hosts and cultural contexts, leads to a similar outcome: the prevalence of virulent isolates. These results strongly suggest an important role of the va gene in the modification of PVY populations.