Investigation on Flavescence Dorée in North-Western Italy Identifies Map-M54 (16SrV-D/Map-FD2) as the Only Phytoplasma Genotype in Vitis vinifera L. and Reveals the Presence of New Putative Reservoir Plants

SIMPLE SUMMARY: Flavescence dorée is one of the most important diseases of grapevines. It is associated with phytoplasmas, pathogenic bacteria transmitted from vine to vine by the insect vector Scaphoideus titanus. The disease spread is limited by eradication of affected vines and control of S. titanus populations using insecticides. Recent studies highlighted that epidemic (transmitted by S. titanus) and non-epidemic phytoplasma strains can infect grapevines, and other insects and additional host plants are involved in Flavescence dorée epidemiology, increasing the risk of disease outbreaks. Thus, the objective of the present study was to identify the phytoplasma strains infecting grapevines in north-western Italy and investigate the presence of additional hosts. Only one epidemic phytoplasma strain was found in diseased grapevines. Moreover, this and other phytoplasma strains, reported as epidemic strains in previous studies, were found in several additional host plants and insect vectors. These findings reinforced the idea that Flavescence dorée epidemiological patterns are not limited to within vineyards, but they involve the entire ecosystems in which the vineyards are located. ABSTRACT: Flavescence dorée (FD) is the most important phytoplasma-associated disease of the grapevine yellows complex in Europe. Recent studies highlighted a great genetic diversity within FD phytoplasma (FDp) strains and demonstrated that their diffusion is not related exclusively to the pathosystem including Vitis vinifera L. and Scaphoideus titanus but involves additional vectors and reservoir plants. This study aimed to investigate FD epidemiology in north-western Italy, with a particular focus on FDp hosts. During field surveys, leaf samples were collected from symptomatic grapevines and other symptomless plant species, and insects were collected within and around vineyards. Phytoplasmas belonging to the ribosomal group 16SrV were detected and typed using nested-PCR-based amplification and nucleotide sequence analyses of the map gene. All symptomatic grapevines were found to be infected by the FDp genotype M54, prevalent in S. titanus and also identified in other known and newly reported hosts. Interestingly, other FDp strains (M38, M50, M51, M121) and FDp-related strains (M39, M43, M48), never detected in grapevines, were largely identified in several known and newly reported host plants and insects including S. titanus. Such evidence confirmed the complexity of FD ecology, expanding the knowledge on the range of FDp host plants putatively involved in the disease spread.