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For sure, the most famous turning point for European winemaking in the late 19th and early 20th century was the phylloxera epidemic. In France, this small insect, brought from the USA, almost completely destroyed the vineyards, provoking a social crisis after an enormous “winemaking” crisis. Wine production volumes fell sharply, winemakers had nothing to sell, and the missing amount of French wine was completed by Spanish or Algerian (but, of course, without any indication on the label). However, despite the fact that the destruction of vineyards by insects allowed to rethink the principles of viticulture, giving a start to important researches in this area, the phylloxera crisis will remain a tragic event for the European viticulture.
It would seem that American varieties as a rootstock and the use of chemicals in the treatment of vineyards could solve many problems and save vines from subsequent epidemics. However, some diseases manifest themselves and spread in a rather obscure way, and winemakers seek to reduce the use of chemicals on their vineyards in order to respect the environment. In addition, some problems are not well understood to find a long-term effective solution, such as using American vines as rootstock to make European vines immune to phylloxera.
One such example is grapevine yellows (flavescence dorée), which appeared on the European continent in the 1920s and has been rapidly spreading, in particular, in the vineyards of Bordeaux since 2010.
What is grapevine yellows?
This is not yet fully understood dangerous disease of plants (in particular, vines), which is quite difficult to detect and contain. It can cause whole epidemics in the vineyards, or it can manifest itself to a greater or lesser extent from year to year.
The disease is caused by microscopic organism – phytoplasma (or mycoplasma). It is somewhat similar to bacteria, but it has no cell wall. Phytoplasma can be parasitic and cause plant and even human diseases (for example, SARS). In vines, phytoplasmas cause diseases of the conductive tissue – phloem – responsible for transporting products of photosynthesis to parts of plants where this process does not occur (underground parts, flowers).
Grapevine yellows is transferred in two ways:
- In nurseries – transmission can occur between rootstock and scion, when grafting a cutting.
- Insects – leafhoppers Scaphoideus titanus, which feed on vine sap and can transfer phytoplasmas from an infected plant to a healthy one.
- Tools when working in the vineyard (for example, when pruning first infected vines, then healthy ones).
Leafhoppers are responsible for the rapid and large-scale infection of neighboring areas, but young plants infected in nurseries or poorly cleaned tools are responsible for the spread of the disease over long distances.
The primary source of the disease was leafhoppers imported from North America along with specimens of American vines. They were intended to become rootstocks for European vines and save Old World vineyards from phylloxera. (The latter, in turn, was also brought from the United States by winemakers who wanted to make wines from American grapes). So, wanting to solve one problem, European winemakers faced another one, perhaps even more detrimental to vineyards.
Infected vine of red grape variety
How does phytoplasma infection occur and how does it affect vines?
Phytoplasma (also called ” mycoplasma “) is a parasitic microorganism that is not able to exist on its own. To survive and spread, it needs a vector – leafhoppers – and a host – a plant, for example, a vine.
Leafhoppers feed on vine sap, “picking up” phytoplasma from an infected plant. These microorganisms do not have a detrimental effect on the insects themselves, but as soon as the infected leafhopper flies to feed on a healthy plant, it will transmit the parasite to it.
Once in the juice, the phytoplasma spreads throughout the plant, infecting the cells of the phloem, a sap-carrying channel rich in carbohydrates. Thus, mycoplasma forces the plant to use all its strength to grow foliage, while blocking the transport of photosynthesis products to those organs that are not intended for this process and are responsible for other functions. And these are the roots that serve to extract moisture and minerals from the soil, flowers intended for plant propagation, fruits …. It turns out that mycoplasma completely subjugates the plant, which is no longer able to feed itself properly or reproduce itself. Since the transfer of photosynthetic products – carbohydrates – by juice is difficult, they remain in the leaves of plants, making them more attractive to phytoplasma vectors – leafhoppers. It turns out that each infected plant becomes a source of grapevine yellows.
Young vines most often die after infection, while more hardy vines weaken, becoming unproductive and economically unprofitable. In addition, if a vine survive, it will never recover from the disease.
The outcome of the disease depends on the variety, because some of them resist better, and some die rather quickly. The severity of symptoms also varies depending on the variety, but the following signs of infection are always present to one degree or another:
- At the beginning of the cycle – late, compared with healthy vines, budbreak and slow growth of shoots.
- Then the shoots stop growing, and the leaves turn yellow (for white varieties) or redden (for red ones), curl downwards and harden a little.
- If the shoot gave inflorescences, then they dry up, because. their development is not supported by the plant.
- If the inflorescence reached pollination and turned into a small bunch, then the berries will acquire a bitter taste, and then partially or completely shrivel.
- At the end of the season, the shoots themselves become sluggish, bend easily and do not harden in august (in French terminology, this phenomenon is called “aoûtement”)
Leaves on vines of red varieties become red
Leaves on vines of white varieties (here – on Sémillion) become yellow and curl downwards
Shoots on the infected vine remain green untill August, they are not subject of the “aoûtement” phenomenon, do not harden and remain “sluggish”
A bunch, which have stopped ripening on an infected vine
A shriveled bunch on an infected grape
Infected vines are less productive
What are the ways to combat grapevine yellows?
Monitoring the spread of the disease and containing it is not an easy task. The first symptoms appear on the vine only a year later, but the plant is already infected and represents a danger to the entire vineyard. In addition, it may be present near the vineyard or directly on the site if the American rootstock has sprouted or grown nearby as a wild vine. However, American varieties do not show symptoms after infection, so in this case it is impossible to accurately determine whether the vineyard is healthy, even if all plants look healthy.
To date, there is no cure for grapevine yellows. It is only possible to prevent its spread by:
- Treatment of dormant young plants still in nurseries by immersion in hot water (50°C), which will kill mycoplasma and some other parasites.
- Timely and regular monitoring of vineyards and grubbing out of infected vines to prevent outbreaks of the epidemic.
- Insecticide treatment to reduce leafhopper populations.
- Reducing the amount of plant foliage with pruning.
The first method is the most easily applicable, practical and environmentally friendly, but unfortunately, not all nurseries resort to it. The last two are much less effective, especially if the site is surrounded by other vegetation (forest, fruit trees) or by abandoned vineyards. Moreover, chemical treatment is unacceptable in an organic or biodynamic vineyards.
The second method is more difficult and longer to apply, but over time it can lead to a complete recovery of the vineyard, or at least to the containment of the disease.
In France, at the local, regional and national level, there are projects to combat grapevine yellows:
- FREDON is a syndicate that unites land owners (vineyards, fruit groves, vegetable fields…) who want to monitor the health of their plantings and fight against plant diseases. In particular, one of FREDON’s missions is to monitor the spread of grapevine yellows and prevent it.
- GDON (Groupement de Défense contre les Organismes Nuisibles de la vigne, Vineyard Pest Protection Group) is an organization operating in France’s largest protected appellation vineyard. Its purpose is to monitor leafhopper populations, promote their reduction, as well as monitor the spread of grapevine yellows and look for infected vines to destroy them. Thus, GDON seeks to eradicate the disease by simultaneously destruction of its source (infected vines) and its vector (leafhoppers) with minimal use of insecticides.
- Government programs to uproot infected vines
Vines, marked by GDON
In general, these programs and organizations annually observe the vineyards where the presence of the disease has been clearly indicated or suspected, noting diseased vines that need to be grabbed out. Then it is the turn of the winemaker to destroy these vines, and officially, if he does not, he faces significant fines. However, not all winemakers are ready to grab out a part of their plantings for various reasons:
- The vines are still bearing fruit somehow
- The winemaker invests significant sums of money to prevent contamination of his plots, but his neighbors do not do this or the nearby plots are abandoned, which contributes to the constant reinfection of the responsible owner
- Very old vines are valuable to the family as they were planted by parents or grandparents
Will grapevine yellows be the phylloxera of the 21st century? With the development of modern science and technology, we can hope that an effective solution to the new dangerous problem for European vineyards will be found much faster than was the case with phylloxera.