Role of plant-specialized Hyalesthes obsoletus associated with Convolvulus arvensis and Crepis foetida in the transmission of Candidatus Phytoplasma solani'-inflicted bois noir disease of grapevine in Serbia

Bois noir (BN), a grapevine yellows disease induced by Candidatus Phytoplasma solani', is spreading and diversifying in several wine-producing regions of Europe, causing increased yield losses and economic concerns. The planthopper Hyalesthes obsoletus (Hemiptera: Cixiidae) is a major vector of the pathogen that causes BN disease in grapevine. Recent studies suggest that epidemiology of BN is affected by the interaction of specific host races of H. obsoletus with weedy host plants that serve as reservoirs for the pathogen. Since BN is common in grape-growing regions of Serbia, a survey was conducted during 2015-2016 to evaluate the host-plant association and vector role of H. obsoletus in central and eastern regions. We tested populations of H. obsoletus associated with Convolvulus arvensis (field bindweed) a known host for the vector and pathogen and Crepis foetida (stinking hawk's-beard) a newly identified host in southeastern Europe. Molecular phylogenetic analyses of H. obsoletus collected from C. arvensis and C. foetida using the mtDNA COI marker determined that the two populations had a genetic divergence of 1.4%. Transmission trials confirmed the vector role of C. arvensis-associated H. obsoletus and identified C. foetida-associated populations as a new and efficient host-specialized vector of BN. Each of the two host-specialized vector populations transmitted Ca. P. solani' to its original host (C. arvensis or C. foetida) and to grapevine cv. Chardonnay. Multilocus genotyping of the transmitted phytoplasma isolates based on tuf/stamp/vmp1 Ca. P. solani'-specific loci indicated pathogen segregation according to the host-plant association of the vector. All isolates transmitted by C. foetida-associated H. obsoletus were of the STOL genotype (tuf-b/STOL/V2-TA), while the C. arvensis-associated population transmitted several genotypes: tuf-b/Rqg50/V4, tuf-b/Rqg50/V14, tuf-b/Rqg31/V14 and tuf-b/Vv24/V14. This study provides evidence of a new independent BN epidemiological cycle of tuf-b Ca. P. solani' strain in the Balkan region, sourced by C. foetida and transmitted by its associated H. obsoletus population.