The epidemiology of rhizomania in the Pithiviers region of Francediversity, micro-evolution and interaction with cultivars at the field scale

Rhizomania, caused by the Beet necrotic yellow vein virus and vectored by Polymyxa betae, is one of the most damageable diseases of sugar beet. Since the widespread use of resistant cultivars, multiple Rz1 resistance-breaking events have been reported worldwide. Understanding the dynamic of the emergence of resistance breaking isolates and their ability to spread is of the uttermost importance. A field-based strategy was set up to follow the evolution of the disease in the epidemic foci of Pithiviers, in France. With the help of high-throughput PCR-based methods, the wide diversity of the BNYVV populations with high infectious potentials in soils was evidenced, with the presence of mixed viral infections (BNYVV A-, B- and P-pathotype) and the emergence of BNYVV reassortants. Variations according to the sugar beet genotype as well as to the spatial dispersion of the virus strongly suggest micro-evolution of the virus over a single season, as well as the fact that the host plant resistance is able to shape the viral population. The data accumulated over four years provide insights for a sustainable management of the sugar beet resistance. A model for the virus epidemic is proposed.