The wine industry is one of the most important agroindustries, both economically and culturally. In 2018, wine production reached 29.2 billions of liters worldwide, with 50% coming from Spain, France and Italy alone. The International Organisation of Vine and Wine (OIV) estimated the economic value of wine sale in 2018 to be 31 billion euros¹.

Winemaking also carries a rich cultural history in Europe. Looking at Switzerland, we could mention the once-in-a-generation “Fête des Vignerons”, or The Lavaux vineyards which were declared a UNESCO World Heritage Site in 2007.

Flavescence dorée (FD) was first detected in Switzerland in 2004². It is highly contagious, classified as a quarantine disease in Europe and Switzerland. The symptoms appear during the summer :

• The leaves shrivel and curl inwards;
• The grape bunch is small; the berries wilt and dry up.
• The stems remain green instead of lignifying (turning into wood).

The symptoms can vary from one cultivar to the next. Red cultivars (the ones used to make red wine) manifest a reddening of the infected leaves, while white cultivars will become yellow.

Symptoms of Flavescence dorée on Divico (Image A, Chardonne, 8/21/2019) and Chasselas (Images B and C, Chardonne, 9/12/2019)

An infected plant usually dies in a couple of years after the initial infection. It’s transmitted from grapevine to grapevine by the leafhopper Scaphoideus titanus.

As of today, no cure for FD has been found. Once the symptoms appear and the diagnostic test has been run, the plant is uprooted and burned to prevent further contamination. It is only possible to get rid of the phytoplasma in young plants that have not been planted yet, at a very early stage of infection, by bathing them in 50°C water for 45 min (thermotherapy). This method is used in a preventive fashion to prevent the introduction of infected plants into the vineyard.

As of today, no cultivar has exhibited any form of natural resistance. The only way to stall the progression of the epidemic is to set up heavy preemptive measures, in the form of vector control through insecticides, burning of the infected plants and preventive thermotherapy. These methods showed positive results in Switzerland: the first case of FD, which was detected in 2016, in the Canton of Valais was successfully eradicated and no other case has been reported since³.

Flavescence Dorée appeared in southern Europe more than 50 years ago and has gradually been spreading North. In 2017, 58% of the French vineyard (439 500 hectares) was inside the contamination zone⁴. In the Canton of Vaud, Switzerland, the number of diagnostics tests for flavescence dorée has gone from 130 in 2016 to 433 in 2018, a 233% increase in 2 years^{5, 6}. Beside France and Switzerland, FD has been reported in Italy, Spain, Portugal, Austria, Croatia, Hungary, Slovenia and Serbia⁷. In 2015, a 3-year initiative called WINETWORK was funded by the EU to improve collaboration between european countries on the subject of grapevine trunk diseases and FD⁸. They released a number of technical datasheets on the diseases and the methods to contain them, mostly directed at researchers and winegrowers, but also to increase public awareness of the epidemic.

In Switzerland, grapevine plants that are suspected to be infected are sampled by the federal phytosanitary police. Those samples are then sent to the federal center for agricultural research, the Agroscope, for pathogen identification. The standard time between sampling and results is 3-15 days. Our goal is to reduce this time to a few hours. Most importantly, our system should enable winegrowers to perform the tests themselves, removing the need for lab-bound equipment and biological training.

Bois Noir (BN) has been present in Switzerland for much longer than FD. The symptoms are identical to FD, but BN is barely contagious; with no risk of an infected plant spreading the disease to the rest of vineyard, BN is not subject to the heavy control imposed on FD. Both diseases are categorized as grapevine yellowings ("jaunisses de la vigne" in french), in reference to the yellow color they induce in the leaves of white cultivars.

Bois Noir on a plant of Divico (Chardonne, 9/12/2019)

FD and BN are impossible to distinguish based on symptoms alone, and differentiation is carried out using a PCR-based method in the lab⁹. The spread of FD in Europe justifies the need for an inexpensive and fast point-of-care detection system.

Divico leaves infected by Bois Noir (left) and Flavescence dorée (right). Notice that bois noir hadn't spread to the whole leaf at the time of the sampling, but the infected parts of both leaves show the exact same symptoms.

The similarity of the symptoms in both diseases isn’t a coincidence : Bois noir and Flavescence Dorée are caused by pathogens belonging to the same genus of bacterias : the phytoplasmas.

Phytoplasmas infect a large number of plants, including globally significant crops like potato, rice and tomato. Phytoplasmas belong to the class of Mollicutes, bacterias characterized by their lack of a cell wall. This makes them extremely sensitive to osmotic pressure and therefore impossible to cultivate in a cell-free environment. As a result, these organisms and their mechanisms of infection are generally poorly understood¹⁰.

Phytoplasmas live in the phloem cells of the host plant (the phloem is the vein-like structure that transports the products of photosynthesis into the other parts of the plant).

The pathogen causing FD is called Candidatus phytoplasma vitis (16SrV ribosomal group). Its vector, Scaphoideus titanus, can acquire the bacteria by feeding on an infected grapevine. Once it’s been ingested, the phytoplasma crosses the intestinal barrier and colonizes the leafhopper’s organs. Once infected, the insect will be able to propagate the pathogen for the rest of its life¹¹.

S. titanus is native of North America and was accidentally introduced in Europe during the XXth century. The leafhopper requires plants of the Vitis genus (e.g. V. vinifera, V. labrusca, V. riparia…) for the completion of its life cycle¹⁰.

Bois noir, known as stolbur in the other crops it infects, is caused by the phytoplasma Candidatus phytoplasma solani of the 16SrXII group. Its vector is the leafhopper Hyalesthes obsoletus. The pathogens usually resides in the stinging nettle (Urtica dioica) and the field bindweed (Convolvulus arvensis)¹². During its erratic search for food, H. obsoletus can sometimes find itself feeding on plant from a different species. This way, the pathogen can get passed on to a variety a hosts, including blackberry, lavender and, of course, grapevine.

However, unlike FD, grapevine is a dead-end for the Bois Noir phytoplasma : Candidatus P. solani cannot infect any of the insect species that feed regularly on the vine. Without a vector, the disease cannot be transmitted to another plant and dies with the host.

¹ OIV (International Organisation of Vine and Wine), 2019, 2019 Statistical Report on World Vitiviniculture, [link to article]

² Jermini M., Schaerer S., Colombi L., 2014, La flavescence dorée et sa gestion dix ans après sa découverte, Agroscope

³ Agroscope, 2019, Flavescence dorée, le Valais exempt mais la lutte continue dans le Lavaux et au Tessin,

⁴ Direction générale de l’agriculture, de la viticulture et des affaires vétérinaires, 2018, Rapport phytosanitaire 2018 – Organismes nuisibles réglementés

⁵ Département de l'économie et du sport, Service de l'agriculture et de la viticulture, 2016, Rapport phytosanitaire 2016 - Organismes nuisibles réglementés et autorisations spéciales

⁶ Groupement de Défense contre les Organismes Nuisibles, Epidémie de flavescence dorée en Europe, [link to article]

⁷ Jeger M. et al, 2016, Risk to plant health of Flavescence doree for the EU territory, p. 13, [link to article]

⁸ http://www.winetwork.eu/

⁹ EPPO protocol for diagnostic of flavescence dorée, 2015, PM 7/079 (2) Grapevine flavescence doree phytoplasm, [Link to article]

¹⁰ Chuche J., 2010, Comportement de Scaphoideus titanus, conséquences spatiales et géographiques, Université de Bordeaux, [Link to article]

¹¹ Chuche J., Thiéry D., 2014, Biology and ecology of the Flavescence dorée vector Scaphoideus titanus: a review

¹² Kessler S. et al, 2011, Host plant preferences of Hyalesthes obsoletus, the vector of the grapevine yellows disease ‘bois noir’, in Switzerland

Unless stated otherwise, the photos were taken by Luc Gabel and now belong to the public domain.