RNA viruses are described as dynamic populations (Domingo, 2002). Indeed, the characteristics of these viral populations can be modified by several processes, including the selection pressure imposed by the host. Potato virus Y (PVY, Potyvirus genus) is one of the most variable plant RNA viral species (Shukla et al., 1994). The filamentous and flexuous PVY particle contains a viral genome which consists of a single-stranded positive-sense RNA molecule of about 10 kb in length. A VPg protein is covalently attached at the 5′ end of the RNA molecule and a polyadenylated tail at the 3′ end. The viral genome includes one large open reading frame (ORF), which codes for a polyprotein cleaved into nine products by three viral proteases, and a second short ORF (PIPO; Chung et al., 2008) embedded within the previously described large ORF. PVY, transmitted in a non-persistent manner by more than 40 aphid species, has a wide host range including cultivated (e.g. potato, tomato, tobacco and pepper) and wild species of the Solanaceae family (Singh et al., 2008). PVY is both one of the most economically important plant viruses and one of the most damaging viruses affecting tobacco and potato crops (Blancard, 1998; Valkonen, 2007). PVY infections can induce necrotic symptoms on infected tissues (e.g. tobacco leaves) and organs (e.g. potato tubers) that can reduce yield and product quality (Le Romancer et al., 1994; Verrier et al., 2001). To limit the impact of PVY in tobacco fields, breeders since the early 1980s have produced cultivars resistant to PVY infection. Three allelic forms (0, 1 and 2) of the recessive resistance gene va, conferring different degrees of resistance to the necrotic symptoms induced by PVY infection, have been identified (Yamamoto, 1992; Blancard et al., 1995) and introduced into cultivars of Nicotiana tabacum (Ano et al., 1995). The va gene does not prevent viral infection of plants but (i) limits cell-to-cell movement of viral particles during the systemic infection of the host (Acosta-Leal & Xiong, 2008) and (ii) reduces the development of vein necrosis symptoms induced by PVY on tobacco leaves (Verrier & Doroszewska, 2004). Even though tobacco cultivars with the recessive resistance va gene represent the most reported genetic resistance source against PVY in this host species (Blancard, 1998), few data are available on the impact of the deployment of the va alleles on the structure of PVY populations. Field surveys have described the presence of virulent PVY isolates on va-derived tobacco genotypes (Latorre & Flores, 1985; Blancard et al., 1995; Verrier & Doroszewska, 2004). Virulence refers here to the genetic ability of a pathogen to overcome a genetically determined host resistance and cause a compatible interaction (Shaner et al., 1992). A recently published French field survey reported both the prevalence of PVY isolates able to overcome the resistance mediated by each of the three alleles va0, va1 and va2 and a reduction in the biological diversity of PVY isolates collected in va tobacco plants (Lacroix et al., 2010). For the last three decades, the culture of French tobacco crops has been based on intensive deployment of the alleles 0 and 2 of the va gene. Indeed, 77·5% of the tobacco cultivated by farmers in 2006 corresponded to va-derived tobacco genotypes (J.-L. Verrier, unpublished data). Such an intensive use of a resistance gene can lead to the emergence of variants putatively more virulent and/or aggressive than the parental viral entities (Pelham et al., 1970; Fargette et al., 2002; Garcia-Arenal et al., 2003; Chain et al., 2007). However, both abiotic and biotic factors (e.g. climatic parameters, the composition of the local host community) can also influence the strength and direction of evolution that drives host–parasite interactions (Thompson & Cunningham, 2002; Laine, 2009). Consequently, a 30-year period of massive deployment of the va gene and/or environmental characteristics of French tobacco-growing areas could have both influenced the modification of PVY population structure. To test the impact of these parameters on the virulence of PVY, biological properties of isolates collected in a contrasted environment need to be studied and compared to previously published results associated with PVY isolates collected in France. Brazil, the second main tobacco leaf producer in the world (http://www.fao.org), is an appropriate environment for such a study because the deployment of va-resistant cultivars, which constitutes the only resistance source used to limit PVY expansion in tobacco fields in Brazil, is both more recent (one decade) and limited (33% of the tobacco crop in 2009, C. Lorencetti, unpublished data) than in the French tobacco context. The main tobacco production in Brazil is situated in the south of the country (376 000 ha in 2009; C. Lorencetti, unpublished data). In addition, PVY has been frequently reported in this region on the basis of samples with symptoms collected from either tobacco or potato plants (Verrier & Doroszewska, 2004; de Avila et al., 2009). Thus, biological properties of PVY isolates present in tobacco samples collected in Brazil were studied and the resulting data compared to previously published results associated with PVY isolates collected in France.