Vine diseases
Grapevines are one of the crops most susceptible to disease in France. Every spring, winegrowers face the same challenge: protecting their vineyards from pathogens that take advantage of the first warm, humid weather to take hold. Fungi, oomycetes, ectoparasites, and wood diseases threaten both the year’s harvest and the long-term health of the vines.
The cost of inaction is high. An uncontrolled downy mildew outbreak can destroy 80% of a crop. Esca, on the other hand, takes several years to establish itself before killing the vine. Quickly identifying the symptoms, understanding the conditions that favor the disease, and choosing the right treatment at the right stage: this is where the bulk of the vineyard’s disease protection comes into play.
This guide reviews the six major fungal and parasitic threats that French winegrowers monitor each growing season, including the causes, characteristic symptoms, and appropriate treatments for each.
Downy Mildew: Symptoms, Causes, and Treatments
Downy mildew is undoubtedly the most dreaded fungal disease in French vineyards. Its pathogen, Plasmopara viticola, is not a fungus in the strict sense of the word, but an oomycete—a microscopic alga that acts as an obligate parasite of the grapevine.
Its life cycle begins in the winter. Oospores overwinter in fallen leaves on the ground. In the spring, when temperatures rise above 11 °C and the first rains arrive, they germinate and release infectious spores onto the young parts of the vine. This is known as the “3-10 rule”: 10 mm of rain, a minimum temperature of 10 °C, and 10 cm of new growth on the vine are enough to trigger an alert. Infections then occur every two to three weeks under humid conditions.
On the leaves, the first symptoms appear as translucent, oily spots that are visible when held up to the light. Under humid conditions, a white, cottony coating appears on the underside of the leaves: this is the pathogen’s sporulation. These spots turn yellow and then brown; the leaf becomes necrotic and falls off. On the clusters, infection before veraison causes the berries to dry out and mummify, a condition often called “gray rot.” After veraison, downy mildew causes “brown rot,” with the berries turning brown, shriveling, and falling off.
The treatments are based on two product families:
- Copper-based contact fungicides (Bordeaux mixture, copper hydroxide): Effective as a preventive measure, they form a protective film on plant parts. European regulations limit applications to 4 kg of metallic copper per hectare per year, calculated as a rolling seven-year average. They remain essential in organic farming.
- Systemic or translaminar fungicides: phenylamides (metalaxyl), QoI (strobilurins), CAA (mandipropamid, dimethomorph). These products penetrate plant tissues and remain active after infection. Be aware of resistance, particularly to strobilurins, which are now widespread in some vineyards. Rotating between chemical classes is essential.
The first treatment should be applied as soon as the “3-10 rule” threshold is reached, before any visible infection occurs. The flowering stage is the most critical time: any failure in protection at this stage can jeopardize the entire crop.
Powdery Mildew (White): Symptoms, Causes, and Treatments
Powdery mildew, unlike downy mildew, is an ectoparasite: it develops on the surface of the vine’s organs without penetrating deeply into the tissues. Its pathogen is the fungus Erysiphe necator (formerly Uncinula necator), which overwinters in the buds or in the form of cleistothecia on the bark.
The conditions that promote its growth differ from those of downy mildew. Powdery mildew does not require rain to develop: it thrives in mild weather, between 20 and 27 °C, with high relative humidity but no precipitation. Shade and poor air circulation within the foliage create the ideal microconditions for its proliferation.
The symptoms are characteristic. A grayish or ash-colored, ash-like coating covers the leaves, twigs, and berries. On the leaves, the affected areas become necrotic and may cause deformities. On berries, infection before the berries close is particularly severe: the hardened skin can no longer keep pace with the growth of the pulp, causing the berries to burst and allowing Botrytis cinerea to enter. Clusters infected late in the season develop a characteristic fungal odor.
The standard treatment remains sulfur, which has been used for over a century. It is applied as a wettable powder or as a dusting treatment, from budbreak through veraison. It acts preventively, and its effectiveness depends on temperature: below 18 °C, it is not very active; above 35 °C, it can cause burns on the berries. As a supplement or in rotation, sterol biosynthesis inhibitors (SBIs), such as triazoles and piperazines, offer curative action limited to 48–72 hours after infection. Again, resistance to SBIs has been documented in several French wine-growing regions, which necessitates the careful management of treatment programs.
Black Rot: Symptoms, Causes, and Treatments
Black rot is caused by Guignardia bidwellii, an ascomycete fungus native to North America that was introduced to Europe with American grapevines in the 19th century. It affects all the green parts of the vine, but the most dramatic damage occurs on the grape clusters.
The disease develops at temperatures between 9 °C and 32 °C, with an optimum range of 20 to 26 °C, and requires a prolonged period of moisture on the plant parts. Primary infections originate from pycnidia and perithecia that form in winter on mummified berries and infected wood. The fungus is active from budbreak through veraison.
On the leaves, black rot appears as circular reddish-brown spots, surrounded by a yellow border and dotted with black pycnidia that are clearly visible to the naked eye. On the twigs and tendrils, elongated black spots appear. On the berries, the disease progresses rapidly: after an initial brownish spot, the berry turns completely black and mummifies within a few days, remaining attached to the cluster. These “mummies” are the primary sources of inoculum for subsequent years.
Management relies on rigorous sanitation: removal of mummified fruit, debudding to improve airflow, and maintaining a balanced load per vine. In terms of chemical control, fungicides effective against downy mildew are often also effective against black rot (copper, IBS, QoI). Early treatments, from budbreak through berry set, are the most critical.
Wood diseases (Esca, Eutypiosis, Black Dead Arm (BDA))
Wood diseases pose a different kind of threat: they develop gradually over several years and compromise the longevity of the vines. Unlike leaf diseases, they cannot be treated with fungicide sprays. Managing them is a multi-year process and relies primarily on prevention and proper pruning.
Esca is a complex of fungi (including Phaeomoniella chlamydospora and Phaeoacremonium minimum) that colonize old wood. Foliar symptoms appear in the summer as “striping”: yellowish or reddish patches between the veins, with a distinct border. When cut, the wood has a soft, white or brown core surrounded by a necrotic zone. The apoplectic form, known as “heatstroke,” causes the foliage to dry out suddenly and completely within a few days. There are no approved products effective against esca in France. Prevention involves pruning to minimize wounds and protecting pruning wounds with approved wound-healing agents.
Eutypiosis is caused by Eutypa lata, a fungus that enters through pruning wounds. The symptoms are characteristic: stunted shoots, deformed and curled leaves on the affected canes, with “V”-shaped necrotic areas visible in cross-sections of the wood. Preventive control relies on applying wound-healing products immediately after pruning, before the rains begin. As a curative measure, vine surgery—which involves excising the affected areas—can save some vines.
Black Dead Arm (BDA), caused by Botryosphaeria obtusa, results in localized necrosis of the wood and the drying out of affected branches. Foliar symptoms resemble those of esca, which complicates field diagnosis. ISAGRI, a provider of agricultural management software, offers tools for tracking phytosanitary treatments that allow winegrowers to monitor the progression of these diseases plot by plot over several growing seasons, facilitating decision-making in the event of a gradual outbreak.
Proper pruning of the vines remains the most effective preventive measure against these three diseases: limit the number and size of wounds, prune during dry weather, and always protect significant cuts.
- Treatment: Choosing resistant grape varieties and quality plants is essential to reduce the risk of infection. Managing plant debris and eliminating diseased vines also limits sources of inoculum.
Revising pruning methods to minimize wounds and respect sap flow helps prevent pathogens from entering the vineyard.
In the event of infection, curettage and regrafting can save partially affected vines. For dead vines, uprooting and replanting are common but costly measures, with the risk of reinfection.
Excoriosis
Excoriose is caused by Phomopsis viticola, a fungus that attacks the herbaceous parts of the vine early in the growing season. It is often underestimated because its early symptoms go unnoticed.
As winter ends, the first shoots develop black spots on the lower internodes, sometimes accompanied by lesions on the leaves. Affected shoots remain spindly and can break easily due to the wood becoming brittle. By the end of the season, infected wood takes on a discolored, whitish appearance.
Treatment is carried out at the “2–3 fully expanded leaves” stage, using fungicides containing folpel or dicarboximides. A second application may be made at the “5–6 leaves” stage in high-risk situations. Agronomic practices complement chemical protection: removal of infected canes and ensuring good airflow at the base of the plant.
Botrytis
Gray mold, caused by Botrytis cinerea, is an opportunistic disease. It mainly affects berries that are damaged or weakened by other factors (powdery mildew, leafhoppers, insect bites) and thrives in cool, damp conditions toward the end of the season.
The characteristic symptom is the gray-brown felt-like coating that covers the affected clusters, resulting from massive sporulation of the fungus. Botrytis has difficulty penetrating healthy berries with intact skins; this is why prevention primarily involves controlling other diseases and mechanical damage. Improving airflow through the canopy by removing leaves around the clusters, managing irrigation, and controlling vine vigor significantly reduce disease pressure.
As part of the treatment regimen, specific anti-Botrytis fungicides (SDHI, anilinopyrimidines) are applied at key growth stages: cluster closure and the onset of veraison. Resistance to anilinopyrimidines and SDHIs has now been documented in several regions, making it necessary to rotate between chemical classes and limit the number of treatments per growing season.
FAQ on Vine Diseases
How can you tell the difference between downy mildew and powdery mildew on leaves?
Both diseases affect the foliage, but their visual characteristics are distinct. Downy mildew produces translucent, oily spots on the upper surface, with a white, cottony coating on the underside, but only under humid conditions. Powdery mildew, on the other hand, forms an ashy coating visible on both sides, regardless of rain. Another useful clue: powdery mildew gives off a characteristic smell of fresh mushrooms. If you’re unsure in the field, simply turning the leaf over is usually enough to tell them apart.
At what time of year are grapevine diseases most active?
The peak risk period is between budbreak (late March to early April, depending on the region) and cluster closure (July). It is during this window that the green parts of the vine are most vulnerable and weather conditions favor infection. Downy mildew and black rot are particularly aggressive between May and June. Powdery mildew, on the other hand, can be severe until August in hot, dry weather. Botrytis, conversely, becomes a problem toward the end of the season, as harvest approaches, when the berries ripen and the nights grow cooler.
Are there any grape varieties that are naturally resistant to disease?
Yes. Varieties known as “PIWI” (from the German pilzwiderstandsfähig, meaning “fungus-resistant”) have been selected for their tolerance to downy mildew and powdery mildew. In France, grape varieties such as Floréal, Vidoc, Voltis, and Artaban are now listed in the official catalog. Their resistance is not absolute: high disease pressure or genetic resistance bypassed by certain strains of the pathogen may require occasional treatment. However, these grape varieties significantly reduce the number of treatments required, which is a major advantage for vineyards in the process of converting to organic farming.
Can vine diseases be treated in organic farming?
Yes, with certain restrictions. Copper (Bordeaux mixture, copper hydroxide) remains the standard treatment for downy mildew and black rot in organic farming, within the regulatory limit of 4 kg/ha/year on average over seven years. Sulfur effectively controls powdery mildew. Preparations based on clays, plant extracts, or microorganisms (such as Bacillus subtilis against botrytis) round out the available arsenal. Agronomic practices—such as leaf removal, aeration, and vigor management—remain essential for limiting disease pressure and reducing the use of inputs, even those that are authorized.
How many fungicide treatments are needed per season?
It depends on the grape variety, the region, disease pressure, and the production system. In conventional viticulture, a comprehensive treatment program can involve 8 to 15 applications between budbreak and veraison. In organic farming, the number is often comparable, or even higher, due to the shorter residual effects of approved products. The goal is not to treat less at all costs, but to treat at the right stage, with the right product, to prevent resistance and limit costs.
What weather conditions promote fungal diseases?
The combination of humidity and mild warmth is the main trigger. A temperature above 10–11 °C, combined with rain and high relative humidity, creates ideal conditions for downy mildew. Powdery mildew prefers dry but warm periods, with cool nights that maintain nighttime condensation on the leaves. Generally speaking, a mild and rainy spring consistently triggers a surge in fungal diseases, regardless of the wine-growing region. Monitoring the plant health bulletins (BSV) published by regional chambers of agriculture allows growers to anticipate risks by production region.
