Competition among Flavescence Dorée Phytoplasma Strains in the Experimental Insect Vector Euscelidius variegatus

SIMPLE SUMMARY: Flavescence dorée (FD) is a serious disease of grapevine, spread in Europe and caused by phytoplasmas. They are uncultivable bacteria, transmitted from plant to plant by hemipteran insects (mainly leafhoppers) and classified according to their genetic traits. Two different phytoplasma strains are associated with the disease, namely FD-C and FD-D. The former outcompetes the latter during the infection of an experimental plant host (periwinkle), although the latter is more abundant in vineyards. Mixed infections are rare in the field. Here, competition between FD-C and FD-D pathogen strains was investigated during the infection of the laboratory insect vector Euscelidius variegatus (Hemiptera: Cicadellidae). Although insects were forced to acquire both strains, single infection, irrespective of the strain type, was more frequent than expected, probably due to competition among strains. Management of the disease mainly relies on the use (compulsory in some European areas) of insecticides, with evident undesirable effects on the environment and public health. Deciphering mechanisms regulating the epidemiology of FDp strains may pave the way towards the integrated management of the disease, such as by fine-tuning the treatments and identifying mild suppressor strains to outcompete the severe ones. ABSTRACT: Phytoplasmas are plant pathogenic wall-less bacteria transmitted in a persistent propagative manner by hemipteran insects, mainly belonging to the suborder Auchenorrhyncha (Fulgoromorpha and Cicadomorpha). Flavescence dorée (FD) is a quarantine disease of grapevine, causing great damage to European viticulture and associated with phytoplasmas belonging to 16SrV-C (FD-C) and -D (FD-D) subgroups. FD-C and FD-D strains share similar pathogenicity, but mixed infections are rare in nature. To investigate the competition among FDp strains, specimens of the laboratory vector Euscelidius variegatus (Hemiptera: Cicadellidae) were forced to acquire both phytoplasma haplotypes upon feeding on FD-C- and FD-D-infected plants or after the injection of both strains. The pathogen colonization of insect bodies and heads was monitored with multiplex qPCR, and the efficiencies of phytoplasma transmission were estimated. Single infection, irrespective of strain type, was more frequent than expected, indicating that competition among FD strains occurs. Hypotheses of competition for resources and/or host active sites or the direct antibiosis of one strain against the other are discussed, based on the genetic complexity of FDp populations and on the high genome variability of the FD-D strain. As FD management still mainly relies on insecticides against vectors, the characterization of FDp haplotypes and the description of their epidemiology also have practical implications.