The specificities of the French vineyard linked to its geographical and therefore climatic diversity, to the diversity of its grape varieties, and to its historical situation as a world crossroads of trade, have led to the development of numerous diseases, which are also distributed in most of the vineyards of the world.
This chapter presents the main characteristics of each disease. For more detailed information, the reader should refer to specialized books and ad hoc websites that provide symptomatological, epidemiological, physiological and agronomic information on each disease. Indeed, it is as important to inform the researcher, the teacher, the student as it is to inform the engineer and the winegrower, because the better the dissemination of information will be the better the fight against these viticultural problems will occur.
In France, there are 1300 clones, 450 varieties and 31 rootstocks approved and registered in the commercial catalog, which means that there are as many possible combinations and therefore as many sensitivities, tolerances or resistances to diseases.
VIRAL DISEASES
Viruses as disease-causing agents are responsible for considerable reductions in the quantity and quality of crops. In France, two groups of viruses, nepoviruses and closteroviruses, are widely encountered on grapevines and induce diseases such as grapevine fanleaf and grapevine leafroll, which are regulated viral diseases.
Grapevine fanleaf
Considered as the most important viral disease in viticulture, short-novel disease covers about two thirds of the French vineyard, one third of which is severely affected. In the past, it was assumed that the disease developed with the fog and the infected vines were said to be “brumards”. Since 1958, it has been demonstrated that this disease is induced by viruses of the nepovirus family (16 currently known in the world). We know the GFLV (Grapevine FanLeaf Virus) and the ArMV (Arabis Mosaic Virus), although less widespread. These viruses are respectively transmitted by two small (<3mm) worms called nematodes, Xiphinema index and X. diversicaudatum. These worms live deep in the vineyard soil and can remain alive and infected with the viruses for up to four or more years after an uprooting. The nematodes bite the vine roots to feed, and can inoculate virus particles during their meals: they are vectors.
Prior to the discovery of the role of Xiphinema index, up to ten years of soil rest with alternative crops was recommended between the removal of an old vine and the planting of a new one.
Viruses cause a wide range of symptoms to the vine. There is a general stunting of the plant, as could be induced by cold, Esca, a poorly adapted rootstock, root asphyxia, or the action of specific hormones. The first symptoms appear on the leaves and are progressive from year to year, as leaves become asymmetrical, variegated, etc. (variegation of the leaves visible from afar); the shoots are then attacked (shortened internodes, double nodes, flattened shoots, etc.); the bunches show coulure and millerandage in greater quantity than on healthy stocks. It can even happen that severely affected vines become practically sterile. The yield can be reduced by 50% and the longevity of the plant is also reduced. To date, there is no curative solution for an infected plot.
At present, the fight against Court-noué is based on sanitary selection and soil disinfection using nematicides. The success and effectiveness of this approach, especially on light soils, are however subject to debate in view of environmental concerns that are increasingly limiting its use, or even leading to its prohibition. In addition to the cost of sanitary selection and the risks of groundwater pollution by nematicides, which are highly toxic products, this strategy is not always effective.
Soil rest has always been recommended after a grubbing up, however the duration before replanting has been reduced over time. We are now talking about two to seven years depending on the soil and agronomic conditions. It is necessary to proceed to a chemical devitalization and an extirpation of the roots. In the Bordeaux region, X. index is dominant in the soil, but 50% of the plots sampled have a low nematode infestation, and would only require a rest of less than two years following uprooting. It is therefore a combination of prophylactic measures that can reduce the contamination of newly planted plots.
Current advances are focused on biocontrol strategies through a better knowledge of the biology of the nematode vector, particularly its survival and vectorial capacity, in order to counter its dissemination in the soil. At the same time, scientific and technical research has focused on developing virus-free plants. In fact, it was illusory and useless to only consider soil disinfection if the plants used for replanting were themselves contaminated by the virus. It was necessary to set up vine propagation methods that would provide grape growers with plant material free of the short-distance virus. Clonal selection was one of these methods. It led to a selection of grafts on one hand, and rootstocks on the other, which were obtained either by sanitary selection from old plots or by a method called thermotherapy. Recently, an approval of a rootstock delaying the contamination has been obtained, however a lesser vigor of the vines is observed. This strategy must therefore be chosen according to the viticultural situation. Thus, nowadays, we have rootstocks and grafts that are not infected and effective means to disinfect the soil; however, there are new approaches that are still being studied; we can mention the planting of nematicidal plants on fallow lands. In Bordeaux, about thirty species have been tested in three phases (laboratory and vineyard) with inoculations of viruliferous nematodes. Among these species, white lupine, red clover, Tagetes minuta, sainfoin, birdsfoot trefoil, hairy vetch, oats or alfalfa, each showed a reducing action on Xiphinema populations during the first phase. The most promising species in the field trials are alfalfa and oats. In 2021, the identification of a targeted resistance of the Riesling grape variety against the virus has brought a considerable advance in the fight against the disease at short-term. Indeed, this resistance factor could be integrated into the genome of rootstocks, and thus be used not only in Alsace, but also in the rest of the world. These different strategies for controlling short-distance dwarf disease are considered within the framework of integrated management, including vegetative rest of the soil, devitalization and planting of healthy vine material.
Viral leafroll
This disease is expanding in France, and particularly affects northern regions, with incidence levels sometimes reaching 70% (Champagne, Beaujolais). Viral leafroll is caused by viruses belonging to the closterovirus and ampelovirus groups. By infecting the vine tissues, these viruses prevent the sap flow. The consequences of a contamination are multiple: a curling and a discoloration of the leaf blade, a modification of the levels of anthocyanins and polyphenols, as well as a decrease of the sugar level in the must. Yield losses can be as high as -40%, but the perception of leafroll varies from vineyard to vineyard and from variety to variety. There are currently nine known strains of leafroll virus in the world, three of which are present in the French vineyard. Among these, strains 1 and 3 (ampelovirus) are the most common and virulent in the northern and south-eastern vineyards, and strain 2 is prevalent in the Loire Valley and Bordeaux regions. The main transmission of leafroll is through wood and plant material during propagation by cuttings and grafting. It is a long distance transmission. However, at the plot level, mealybugs (insects) are the most efficient vectors of these viruses to grapevine, especially mealybugs such as Phenacoccus aceris and Heliococcus bohemicus and the well known soft scale Parthenolecanium corni. In Europe and in France, virus leafroll is subject to sanitary selection and certification regulations. As for all virus diseases, clonal and sanitary selection does not allow to introduce the disease at planting. A control by registered insecticide targeted to mealybug immatures is possible if positioned at the time of treatment against the second generation of grapevine moths. Currently, control strategies based on biological control, either by voluntary action or by preservation of auxiliary fauna (ladybugs, lacewings, arachnids, parasitoids) (conservation biological control), are being studied to limit the expansion of scale insects in the vineyard.
BACTERIAL DISEASES
Like viral diseases, bacterial diseases of grapevines have very heterogeneous epidemiological, etiological and symptomatic characteristics. A distinction is made between diseases caused by bacteria with a cell wall (broussins, bacterial necrosis), and diseases caused by bacteria lacking a cell wall, commonly called “phytoplasma yellows” (flavescence dorée, bois noir). Grapevine yellows induced by phytoplasmas are by far the most problematic bacterial diseases. The main reason is the crucial role of airborne insect vectors in the transmission of phytoplasmas which makes control difficult.
Phytoplasma yellows
Flavescence dorée (FD) and bois noir (BN) show an identical range of symptoms on grapevines. However, only FD causes the death of the vine. These symptoms are:
– a sectorial discoloration of the leaves taking on a yellowish tint in white grape varieties and a reddish tint in red ones;
– a hardening of the leaves becoming brittle with rolled edges;
– a discoloration of the main veins tending towards a drying of the blade;
– a non or bad lignification of the branches taking the consistency of rubber (weeping habit);
– a drying of the inflorescences;
– a withering of the berries whose fibrous and compact pulp becomes bitter.
The flavescence dorée
It has been nearly 70 years since the discovery of flavescence dorée in the vineyards of Gascony and for the first time in the world. Since then, this phytoplasma disease has colonized half of the French vineyards, going up through the Rhone valley, then spreading to most of the European vineyards. Flavescence dorée is a quarantine disease subject to mandatory reporting. Since the middle of the 1980s, the French authorities have issued decrees obliging wine growers to declare cases of contamination and to ensure effective control of the insect that propagates this plant pathogen. The target of the control is an insect vector called leafhopper, or Scaphoideus titanus, which carries from vine to vine, a phytoplasma that infects the vine cells. The genetics of this phytoplasma is complex, with several strains depending on the country and region. Moreover, this agent has been detected not only in grapevines, but also in rootstocks adjacent to plots, and in wild plants such as clematis and alder, without all these species expressing symptoms (we speak of healthy carriers). In 2020, a team of researchers from Bordeaux, within a European consortium, demonstrated the origin of the flavescence dorée phytoplasma. Contrary to the dogma of an American origin of the phytoplasma, as well as the Scaphoideus leafhopper, this phytoplasma is in fact European, and infested to the alder tree. The American leafhopper would then have taken hold of it after its introduction into Europe (1950s) and would have transmitted it to the vine. This highly mobile leafhopper, with only one generation per year, is a strict ampelophagous species, accomplishing its entire development cycle on grapevines, and has successfully propagated the phytoplasma throughout the European vineyards.
This epidemic disease is very worrying because it affects all grape varieties, with however variable expressions of symptoms; Chardonnay is very sensitive, Merlot much less so. It can be as disastrous for vineyards as phylloxera was at the end of the 19th century. Currently, the only way to fight against the spread of this disease is to uproot the diseased vine stocks and to destroy the insect vector (larvae and adults), by using several specific insecticide treatments. In contaminated areas, vines, graft nurseries and rootstocks must receive three insecticide treatments. Despite attempts, the search for biological control has not been convincing. A prophylactic measure in force is the treatment with thermotherapy of graftings and rootstocks guaranteeing flavescence dorée free material before planting.
Bois noir
The first reports of bois noir in France coincide with those of flavescence dorée. It is sporadically distributed in the northern vineyards (Jura, Burgundy, Alsace). In recent years, and especially in 2021, bois noir has become the most common yellows disease in Burgundy in all tests for symptomatic strains. The neighboring Jura is also strongly impacted. The etiological agent, as well as the insect vector, are different from those of flavescence dorée. It is a cixiid planthopper (close to leafhoppers) named Hyalesthes obsoletus, which is the main vector known in France and in Europe, even if other leafhoppers in Europe would sporadically participate in a vection of the phytoplasma. It efficiently transmits to grapevines phytoplasma particles related to the stolbur group. Stolbur is an opportunistic bacterium present on many crops, including solanaceous plants (tomato, eggplant, potato). It is carried by insect populations from weeds or reservoir plants (bindweed, nettles, etc.) present in wastelands, borders and interrows. The stolbur is accidentally transmitted by the insect while feeding on the vine. Hyalesthes obsoletus is a mobile insect that does not live on the vine, which makes direct control of the vector impossible. Only prophylactic methods (keeping plots free of relay host plants, winter turning of fallow land with high insect population) are currently recommended.
Bacterial necrosis
Bacterial necrosis is caused by the bacterium Xylophilus ampelinus, which is native to grapevines, and is found in almost all of southern Europe. In France, five wine-producing territories are concerned: Armagnac (Aquitaine and Midi-Pyrénées), Roussillon, Charentes and Diois, i.e. 67 communes listed within nine “appellations”. The nature of the grape varieties, the climatic conditions, the technical itineraries (e.g. pruning and mechanization of the grape harvest) determine the expansion and multiplication of the disease’s foci. The fight against the disease is based on cultural and prophylactic interventions: planting non-contaminated grafted vines, pruning during the resting period of the vegetation, pulling out and burning the stumps and diseased wood. These interventions can be completed by a copper protection (Bordeaux mixture). A prophylactic approach by hot water treatment, as for yellows, is also recommended.
Brushes and crown-gall
These are tumoral structures that form on the surface of organs capable of growing in thickness. They can be found on the aerial part of the vine, on the trunk, the arms or the shoots. The growths are caused by a bacterium, Agrobacterium vitis, and often appear after winter frosts and injuries. Preventive measures (selection of seedlings) and prophylactic measures (protection of wounds) are recommended. Indeed, contamination is mainly through wounds or stomata. If the soil is infected, the slightest wound in the crown is an entry point for bacterial particles that will integrate their DNA into the vine cells. The latter will respond to this contamination by an anarchic cellular multiplication generating an exponentially growing gall.
