Characterisation and comparison of French and Brazilian potato virus Y (PVY) isolates collected from PVY-susceptible or -resistant tobacco plants carrying the recessive resistance gene va
Improved tobacco cultivars possessing alleles of the “va” recessive resistance gene have been deployed worldwide to mitigate consequences of potato virus Y (PVY) infections. Unfortunately, necrotic PVY symptoms were reported, suggesting that PVY is able to overcome this resistance. Field surveys were performed in France (2007) and Brazil (2009) to i) estimate the prevalence of PVY in leaf samples mainly collected on tobacco plants with symptoms and to ii) characterise PVY isolates present in susceptible and va-derived tobacco cultivars. A serological typing procedure was applied using polyclonal antisera raised against the main viral species affecting tobacco in each country, including PVY. In France and in Brazil, viral species were detected in 80.8% and 52.4% of collected leaves and PVY was present in 83.5% and 72.1% of infected samples, respectively. Statistical analysis showed for both France and Brazil that the probability for a plant to be infected with PVY is reduced in va carrying hosts. To define virulence of PVY against alleles 0, 1 and 2 of the va gene, collected isolates (86 from France and 29 from Brazil) were mechanically inoculated on eight plants of each of one susceptible and three va-derived tobacco cultivars. Both qualitative and quantitative analyses showed that most isolates (N=55/86, France, and N=20/29, Brazil) overcome the three va alleles. Moreover, the monitored biological diversity of PVY isolates was higher in susceptible tobacco hosts than in va-derived ones that were mainly infected by multi-virulent isolates. In conclusion, the use of va-derived cultivars in two environments with contrasting climatic conditions, local hosts and cultural contexts, led to a similar outcome: the prevalence of virulent isolates. In a further study (Janzac et al., CORESTA Sapporo 2012), some of the collected isolates were sequenced to check the molecular origin of this virulence.