A genetic analysis reveals low prevalence of phytoplasma infection in Hyalesthes obsoletus Signoret, vector of ‘bois noir’, in SW-Germany

Abstract

Bois Noir is a grapevine disease responsible for severe economic losses in wine production. Bois Noir is caused by Candidatus Phytoplasma solani, cell wall-less bacteria belonging to the taxonomic group 16Sr-XII-A. In Germany, they are known to be vectored from plant to plant by the cixiid Hyalesthes obsoletus, but so far the prevalence of the disease in the vector population, as well as its spatio-temporal distribution is poorly understood. We therefore analyzed infections of H. obsoletus collected in different vineyards in Baden (South-Western Germany) with quantitative real-time PCR. From 125 analyzed individuals, only five were infected with Ca. Phytoplasma solani. All infected individuals were colonized by Ca. Phytoplasma solani type I which is associated with the host plant Urtica dioica (stinging nettle). More research is needed to understand the reasons of this surprisingly low prevalence of Bois Noir in the population of H. obsoletus in South-West Germany.

What the paper shows and why it matters

(AI-generated)

Bois noir is a grapevine disease that causes severe losses in wine production. It is caused by the bacterium Candidatus Phytoplasma solani, which the planthopper Hyalesthes obsoletus carries from plant to plant; the insect’s two most relevant host plants in Germany are stinging nettle and field bindweed, and the bacterium comes in a genetic type linked to each. The disease has become a growing concern in Germany. In the Baden region of south-western Germany, earlier work showed that the insect occurs throughout the region and gave a basic picture of its ecology, but how many of the insects actually carry the bacterium was poorly known. Earlier studies in other regions, and one in Baden in other years, had reported higher infection rates.

The study finds that only five of 125 Hyalesthes obsoletus planthoppers collected in different vineyards in Baden carried the bois noir bacterium, and all five carried the type linked to stinging nettle. The infected insects came from places spread across the region, with no discernible spatial pattern. The paper judges that chance alone cannot reconcile this with a true prevalence of around 20%. It names differences in sampling as a plausible but untested explanation: its sites were chosen at random, whereas the site choice in earlier studies was sometimes not clearly described and may have been close to vineyards where the disease was already known, which would raise the apparent infection rate. The paper stresses that it can only speculate here. A further aim, to compare the body size and weight of infected and uninfected insects, could not be answered because so few insects were infected.

**Figure 1.** Collection sites in the Baden region. From Schiro et al. (2015), *PeerJ Preprints*, licensed under CC BY 4.0. Click the image to enlarge.

The results suggest that infection in the planthopper in Baden may be lower than earlier figures imply. The paper does not decide whether this reflects a real decline of the disease in the insect population or systematic differences between studies. It calls for larger surveys with more insects and more sampling sites, together with analysis of the host plants (stinging nettle, field bindweed and grapevine), to tell the two apart. Limits remain: the text is a preprint that has not been peer-reviewed, and only five infected insects were found.