Mildew, powdery mildew, phylloxera… These names probably ring a bell… But do you know what they are, how to spot them and how to treat them? Let’s go through the basics together!
1. DOWNY MILDEW (PLASMOPARA VITICOLA) also simply referred to as mildew
This is undoubtedly the most widely known vine disease, and probably one of the most common. This fungal disease – or cryptogamic disease, which means caused by the presence of a fungus – is not confined to vines. It can also affect tomatoes or potatoes, for example. This disease originated in America and reached Europe in 1878. It is now present throughout the world. Although it can cause considerable damage to vines, it is treatable.
Causes
Humidity and dampness are the main culprit behind this disease, spread by wind or splashes when temperatures exceed 11°C. The disease is triggered by oomycetes, tiny algae-like fungi.
How to spot downy mildew on vines?
Downy mildew affects vine leaves, causing them to develop brown or white patches, resembling a sort of mould with a cottony texture. If the disease progresses too much, it can lead to not only the leaves rotting, but also the whole vine, the shoots and the berries.
Treatment
The best-known treatment for mildew is the Bordeaux mixture, a combination of copper sulphate, fat lime and water – where copper prevents the germination of mildew spores. In recent years, a number of environmentally conscious winemakers have been trying other remedies to combat downy mildew. These include the use of skimmed milk, which seems to be a promising solution, or the use of specific fungi that inhibit the growth of oomycetes, the pathogens responsible for downy mildew.
2. POWDERY MILDEW (ERYSIPHE NECATOR)
This disease is also caused by a microscopic fungus that originated in North America and first arrived in Europe in 1845 in the mid-nineteenth century. In France, it is most prevalent in the Mediterranean region though it now affects all winegrowing regions, including the northern ones, no doubt as a result of global warming. This disease diminishes the harvest to a greater or lesser extent depending on the scale, and also adversely affects the organoleptic qualities of the wine, producing unpleasant aromas, bitterness and astringency.
Causes
Powdery mildew is caused by a fungus named Uncinula Necator, also known as Erysiphe Necator. This disease is more likely to appear in hot, humid weather, particularly in May and June. It particularly affects young shoots.
How to spot powdery mildew on vines?
This disease can infect any living part of the vine – leaves, bunches and shoots – coating them with a kind of white or grey dust, similar to mildew, and blackening the veins. When it strikes the berries, it appears in the form of grey dust covering them, eventually causing the berries to burst. When the leaves are infected, photosynthesis is impaired, which diminishes the plant’s vitality.
Treatment
Preventive measures include limiting the vigour of the plant (green harvesting, etc.) and treating with sulphur (fungicide) when the disease is present.
3. PHYLLOXERA (DAKTULOSPHAIRA VITIFOLIAE)
This disease naturally reminds us of history lessons, due to the great phylloxera crisis that decimated European vineyards in the second half of the 19th century from 1861 onwards. Phylloxera was brought on by an insect, a devastating aphid that parasitises vines and originates from the United States of America. This is an extremely severe disease, fatal to vines, which came close to decimating the European vineyards in the 19th century. The issue was eventually “fixed” by turning to American rootstocks, which are resistant to the disease.
Causes
The insects sting the grapevine roots, creating infections caused by the poisonous saliva they release into the plant, leading to root rot and, ultimately, the death of the vine within three years.
How to spot phylloxera on vines?
This disease also affects the foliage, causing a yellowing of the leaves and a sort of blistering on the underside. The roots are the most severely affected, with the disease causing nodosities and tuberosities, bumps, irregularities and necrosis.
Treatment
Historically, when the phylloxera crisis struck, French vines were grafted onto American rootstocks resistant to the disease. While many other remedies have been tested, there is no real treatment other than this. It is worth noting, however, that sandy soils are not prone to phylloxera, allowing some vines to remain ungrafted.
4. GREY mould (BOTRYTIS CINEREA)
Botrytis cinerea is caused by a fungus and is both a dreaded disease and a vineyard miracle when it comes to the quest for noble rot in the Sauternes region, for example. Not only does this disease affect vines, but it can also spread to other plants such as tomatoes and strawberries. Botrytis cinerea is a very old disease, known since ancient times, and is one of the most common fungal diseases to affect vines. It decreases yields by rotting the berries.
Causes
This fungus thrives in damp conditions, at temperatures generally between 16°C and 25°C, and near water sources. Any cuts or scraps suffered by the vines (cuts, pruning, etc.) increase the risk of contamination.
How to spot grey mould on vines?
While all parts of the vine, with the exception of the roots, can be affected by this disease, it primarily attacks the fruit. The berries are covered in a kind of grey mould and eventually wither, rot and turn brown, with some falling to the ground. This disease is most visible during veraison when the grapes start to change colour. At other stages of the vine’s development, the disease can also dry out buds, cause flower abortion and leaf dieback.
Treatment
The fight against grey mould involves a combination of preventive measures and curative treatments. Preventive measures include controlled grass cover (sometimes only between the rows to reduce the humidity under the vines), early and moderate leaf thinning to aerate the bunches and a reduction in nitrogen inputs. Otherwise, current treatments are essentially chemical, once again using the Bordeaux mixture. The use of living organisms, such as bacteria and fungi, and natural vine stimulators, is also being tested. Curative treatment consists in reducing the damage and proliferation of the disease by removing affected grape bunches.
5. ESCA
This disease, caused by a parasitic fungus, attacks the wood of the grapevine. One of the oldest vine diseases, it was even a problem in Roman times, and it is becoming increasingly prevalent.
Causes
This disease is caused by fungi spread through the air. It attacks vines by penetrating through fresh pruning or trimming wounds, particularly during mild, wet winters. Generally speaking, the conditions conducive to the growth of this disease are heat, dampness and a lack of oxygen.
How to spot Esca on vines?
The fungus damages the vine’s wood from the inside, causing it to become soft and spongy, and turning it a yellowish colour. Berries and leaves dry out and brown stripes may appear on the leaves along the veins.
Treatment
The Guyot Poussard method of pruning vines appears to be effective in preventing the proliferation of the disease. This is a ‘renewal’ training system, where the fruiting side branches are pruned out every winter, and new ones trained horizontally to replace them and carry the coming summer’s crop. Another promising method is called curettage, which involves removing the affected parts of the vine using a small chainsaw – a sort of surgical operation to remove a tumour! But in many cases, for the sake of simplicity, winegrowers prefer to pull out and replant.
6. FLAVESCENCE DORÉE
Flavescence dorée (from the French “Flavescence”, yellowing and “dorée”, golden) is an extremely serious vine disease first observed in France in the 1950s. First confined to the south of the country, it eventually affected every French wine-making region. Crop losses can be very significant, sometimes the entire harvest can be lost, and in the long term, the affected vine ends up withering away.
Causes
This disease is caused by a phytoplasma (a bacterium with no cell wall carried by leafhoppers, a plant-sucking insect that feeds on plant sap) that develop on the vine.
How to spot flavescence dorée on vines?
Part of the grapevine yellows family of diseases, it is easily be confused with other infections. The foliage changes colour, depending on the grape variety (either reddening or yellowing) when the disease appears, and the leaves curl up, which can also cause the grapes to shrivel. Unfortunately, the symptoms are not immediately apparent, but tend to appear a year or more after the infection starts.
Treatment
In France, measures to prevent flavescence dorée have been mandatory and regulated since 2001. Each year, pest control measures are defined by the French authorities on a prefectural level. Infected vines must be uprooted and burnt as quickly as possible. If more than 20% of the vines in a given parcel are affected, the entire plot will suffer this fate. Winegrowers must also spray insecticides to combat the leafhopper, and the vine’s trunks must be treated with hot water.
7. GRAPEVINE FANLEAF VIRUS
Grapevine fanleaf is a serious viral disease. It developed throughout the world’s vineyards after the phylloxera crisis.
Causes
The disease is caused by a virus (in France, mainly GFLV – Grapevine Fan Leaf Virus – or, more rarely, ArMV – Arabic Mosaic Virus), which is spread by the parasitic nematode Xiphinema index, which punctures the vine’s roots to draw energy from it. Clay soils are the most favourable environment for these nematodes to develop, unlike sandy soils, which don’t allow them to thrive.
How to spot the grapevine fanleaf virus on vines?
Symptoms of this disease include colour changes (yellowing) and shape irregularities (asymmetry, double leaves, irregularly shaped or more deeply indented leaves, growths on the underside, etc.) in the vine’s leaves, as well as wood deformities (shortened internodes, many secondary buds, double nodes, etc.). The disease can also lead to grapes failing to develop after flowering and millerandage (bunches of grapes with different sizes). Affected vines produce less because they are weakened. In the long term, this incurable disease causes vines to wither. The symptoms and their intensity vary greatly from one grape variety to another and from one region to another, which can make it difficult to identify the disease. For this reason, it is advisable to supplement symptom observation with a serological method or biological indexing, for example.
Treatment
There is no curative treatment against the grapevine fanleaf virus. The only option is to take preventive action or limit the damage to plots by pulling out diseased plants. To ensure effective land clearing, the first step is to devitalise the vines by treating them with a systemic herbicide such as glyphosate. Then as many roots as possible must be pulled out, and the contaminated soil must be left to rest for 7 to 10 years before replanting, to eliminate vine regrowth on these resting plots and finally to use new, healthy, certified plant material. Alternatives to herbicides that are less harmful to the environment are currently being tested. These include the use of plants with a nematicidal effect, or the natural resistance of certain grape varieties such as Riesling…
Nematodes can continue to survive for several years after the vines have been pulled up, feeding on the root tips that remain in the soil. They can even enter a dormant state and abstain from feeding for more than five years, only to reactivate when conditions become more favourable for its development.
8. BLACK ROT (GUIGNARDIA BIDWELLII)
This cryptogamic vine disease originated in eastern North America. It arrived in France around 1885 when cuttings were imported to rebuild European vineyards after the great phylloxera crisis. The disease spread throughout the Americas, Europe and Asia. In France, it mainly affects the south-west and west of the country. Although it is not considered a serious disease, it can nevertheless cause considerable damage to vineyards and have a negative impact on the wine made from effected plants.
Causes
This disease is caused by Guignardia bidwellii, a phytopathogenic ascomycete fungus. It mainly strikes when spring temperatures warm up – the fungus becomes active from 9°C up to 32°C, peaking between 20°C and 26°C – and there is plenty of humidity. The fungus is spread locally by the wind, which transports the spores.
How to spot black rot on vines?
This disease targets all the green parts of the vine, from bunches to leaves and branches. Black rot manifests in the form of brown or red stains a few millimetres in size on the leaves, with small, shiny black pustules. The stains and pustules can also affect the branches. Affected berries develop a reddish tinge, wither and dry out.
Treatment
Although there is no specific treatment for black rot, other treatments such as those designed to fight downy or powdery mildew are equally effective against this disease. Organic treatments can also be used, with copper and sulphur-based preparations.
