Contrasting susceptibilities to Flavescence dorée in Vitis vinifera cultivars and progenies suggest segregation of genetic traits involved in disease response

Flavescence doree (FD) is a severe epidemic disease of grapevine in Europe caused by FD phytoplasma (FDp), a small wall-less bacterium transmitted by the leafhopper vector Scaphoideus titanus and classified as a quarantine organism. The mandatory control of the disease consists of eliminating infected grapevines, spraying insecticides against the vector and planting healthy material. Given the economic, environmental and social impacts of such measures, viticulture is awaiting alternatives. After extensive surveys in vineyards, we showed that 'Cabernet Sauvignon' is highly susceptible, with a high proportion of symptomatic branches and high phytoplasma titres, in contrast to 'Merlot'. Localized insect transmissions and grafting experiments showed that phytoplasma circulate in the whole plant in 'Cabernet Sauvignon', whereas, in 'Merlot', they are restricted to the transmission point. Analysis of the parentage of 'Merlot' and 'Cabernet Sauvignon' suggested that 'Merlot' inherited its low susceptibility from its maternal genitor, 'Magdeleine Noire des Charentes' (Mag). This hypothesis was reinforced by preliminary analysis of a reconstructed progeny between the parents of 'Merlot', Mag and 'Cabernet Franc'. Furthermore, we developed an insect-mediated transmission protocol under high confinement, mimicking natural conditions. This allowed the classification of 28 Vitis accessions into three distinct categories, according to the percentage of infected plants and their phytoplasma titres. In Vitis vinifera cultivars, reduced symptoms, low phytoplasma titres and low percentages of infected plants were found to be associated. Rootstocks and their Vitis sp. parents, although displaying high percentages of infected plants and intermediate to high phytoplasma titres, shared a symptomless response. Thus, rootstocks in mother plant parcels, or when they grow wild near to vineyards, are a potential and silent reservoir of contamination that endangers vineyards. Altogether, our data suggest distribution of genetic traits within the genus Vitis involved in insect-mediated phytoplasma transmission, multiplication, circulation and symptom development.