Lavender decline in France is associated with chronic infection by lavender-specific strain of " Candidatus Phytoplasma solani."

Lavender decline compromises the French lavender production, and preliminary data have suggested the involvement of " Candidatus Phytoplasma solani" in the etiology of the disease. In order to evaluate " Ca. P. solani" epidemiological role, a three year survey was conducted in Southeastern France. " Ca. P. solani" was detected in 19 to 56 % of the declining plants, depending on seasons and cultivars, and its prevalence was correlated with symptom severity. Autumn was more favorable than spring for phytoplasma detection, and " Ca. P. solani" incidence was higher in Lavandula angustifolia than in L. intermedia hybrids. Detection of the phytoplasma fluctuated over months supporting the chronicity of infection. Three ' Ca. P. solani' sec Y genotypes S17, S16 and S14 were the most prevalent in lavender fields and were also detected in nurseries, whereas strains detected in surrounding bindweed and wild carrots were mostly of the S1 and S4 genotypes. This suggests lavender is the main pathogen reservoir of the epidemic. Adults and nymphs of the planthopper vector Hyalesthes obsoletus were commonly captured in lavender fields and were shown to mainly harbor the prevalent phytoplasma genotypes detected in lavenders. The " Ca. P. solani" genotype S17 was transmitted to Catharanthus roseus periwinkle by naturally infected H. obsoletus Finally, the inventory of the bacterial community of declining lavenders that tested negative for ' Ca. P. solani' by 16S rRNA deep-sequencing ruled out the involvement of other phloem-limited bacterial pathogens. IMPORTANCE The etiology and main pathways for spread of the lavender decline, an infectious disease affecting the French lavender production since the 1960s, have remained unclear, hampering the development of efficient control strategies. An extensive survey of lavender fields led to the conclusion that " Candidatus Phytoplasma solani" was chronically infecting declining lavenders and was associated to large infectious populations of Hyalesthes obsoletus planthoppers living on the crop itself. Lavender appeared to be the main reservoir host for lavender-specific phytoplasma strains, an unusual feature for this phytoplasma that usually propagates from reservoir weeds to various economically important crops. These results point out the necessity to protect young lavender fields from the initial phytoplasma inoculum coming from surrounding lavender fields or from infected nurseries, and to promote agricultural practices that reduce the development of H. obsoletus vector populations.