Biggest vineyard threats and how to fight them

The path to harvest is never easy, as any viticulturist, winemaker or vineyard worker will know.

From insect attacks, fungal nasties and unpredictable bouts of frost and drought, knowing your powdery mildew from your flavescence dorée is crucial in the fight against some of the most common vineyard threats.

While this is by no means an extensive list of the multitude of diseases and threats that a producer could face, or a robust guide in how to deal with them, it features a potted history of some of the most common, their origins and peculiarities.

Ever wondered why powdery mildew is named such, where the word botrytis came from, or where and for how long the world’s longest, recorded, drought occurred?

Read on for our roundup of some of the most common vineyard threats faced by winemakers, and what can be done to fight them…

Drought

Type: Climactic

What is it: An extended period whereby water is in short supply.

Cause: A prolonged period of below-average precipitation in a specific region.

Impact: Drought can be a benefit to cooler regions that typically receive high levels of rainfall, keeping a vine’s vigour in check and resulting in higher concentration, albeit lower yields. In warmer climates which typically receive less rainfall, such as California and Australia, prolonged drought can harmfully reduce yields, limit growth and ripening of fruit.

Remedy: There is no solution to drought – no one can make it rain. But there are ways to lessen its impact upon a vineyard and manage its effect in the future. Some producers may adopt dry farming techniques, whereby a vineyard is encouraged to rely only on natural annual rainfall for growing grapes. This is easier in regions with higher rainfall such as Bordeaux and Burgundy, but also trains the vineyard to require less water. In hotter countries, where water shortages are frequent, such as California, drip irrigation may be used to control the level of water given to a vine, if dry farming is not an option. Using less water can result in smaller, but more concentrated, grapes, so can be an advantage to those operating within the premium sector, compared to commercial producers that rely on high volumes. In some regions, such as South Africa, winemakers are experimenting with grapes that naturally require less water, making them more future proof to the impact of drought.

Fun fact: The most prolonged drought ever recorded occurred in the Atacama Desert in Chile and lasted 400 years from 1571 until 1971. The Atacama, remarkably, is home to a limited number of vineyards, which are also among the most extreme in the world, having attracted the interest of Viña Ventisquero, among others. The company planted Chardonnay, Sauvignon Blanc, Syrah, Viognier, Merlot and Pinot Noir in 2007 in a southern part of the Atacama Desert called Huasco, and has produced around 1,000 bottles from each variety from the 2011 vintage under the Tara label.

Flavescence dorée

Type: Bacterial (phytoplasma)

What is it: A bacterial disease typically contracted by young vines in the nursery and then spread by insects known as leafhoppers between vineyards.

Origin: The first recorded instance of flavescence dorée was in the Armagnac region in south west France in 1949. Chardonnay and Riesling are among the most susceptible of varieties to the disease, as well as Grenache, Tannat, Pinot Noir and Pinot Gris.

Impact: Can kill young vines and make old vines uneconomical.

Remedy: Insecticide sprays can reduce leaf hopper populations and therefore the spread of the disease. Young vines in the nursery can be dipped in hot water to rid them of any phytoplasma before they are planted in the vineyard, which helps to stem the disease somewhat.

Fun fact: In recent times two winemakers have been acquitted of charges brought against them over their refusal to spray their vines for flavescence dorée. Thibault Liger-Belair, a biodynamic winemaker, was summoned to court by the French Ministry of Agriculture after he refused to spray vines at his vineyard in Beaujolais at Moulin-à-Vent in Beaujolais in 2013, following an outbreak of the disease in Mâconnais. He was acquitted in 2016. Emmanuel Giboulot, another biodynamic winemaker, from Burgundy, was fined €500 in April 2015 for not spraying his vines against the pest, but had his conviction overturned on appeal. He had been facing a maximum fine of €300,000 and six months in prison.

Powdery mildew (oidium)

Type: Fungal

What is it: A disease which results in a fine layer of fungus growing over the grape, appearing as grey-white ash atop tiny stalks which makes it look ‘powdery’, hence the name.

Origin: Powdery mildew is native to North America, but was given the name Odidium Tuckerii after a gardner, Mr Tucker, who first identified the disease on grapes in his greenhouse in Margate, England, in 1845. The disease was later reported in France in 1847. Spores are spread by the wind and can survive over the winter inside buds or on the surface of a vine. 

Impact: Powdery mildew affects the green parts of the vine and results in unripe grapes, lower yields, stunted bud and shoot development and, ultimately, tainted wine. 

Remedy: Sulphur-based sprays can be used to limit the development of powdery mildew.The disease thrives in hot, shaded areas, but humidity plays less of a factor in its spread than other fungal diseases. Exposing bunches to sunlight can help inhibit the growth of spores.

Fun fact: Generally, American vines are more resistant to powdery mildew than European vines, as the disease was originally native to the region.

Grapevine Trunk Diseases (GTDs)

Type: Fungal

What is it: Refers to a number of fungal wood diseases that can infect the trunk of a vine, including black foot and Petri disease, esca, eutypa dieback and Botryosphaeria dieback, also known as black dead arm.

Origin and cause: Microscopic spores are carried by wind and rain and infect a vine through pruning wounds, often during its dormant season. Esca, one of the most common GTDs, is caused by the fungi Phaeomoniella chlamydospora and Togninia minima.

Impact: Causes lower yields and the eventual death of a vine, significantly reducing the potential lifespan of a vineyard.

Remedy: There is no cure, although the now banned pesticide sodium arsenite was known to reduce instances of the disease. Prevention is the best cure. Regular pruning of old wood, burning of pruned wood and wound protection the best course of action. The International Council of Grapevine Trunk Diseases, formed in 1998, meets annually to compare research into tackling these diseases.

Fun fact: Across France, around 13% of French vineyards are estimated to be infected by GTDs, according to the French agriculture ministry and French Wine Institute (IFV), with GTDs lowering potential wine production by 13% in 2014.

Downy mildew

Type: Fungal

What is it: A disease that thrives in warm, humid conditions, caused by the fungus Plasmopara viticola, which is native to North America. Consequently, North American vines are more resistant to the disease than European rootstocks.

Origin and cause: The disease is believed to have been accidentally imported into Europe around 1878, when many American rootstocks were being imported into Europe to help fight phylloxera. The disease is now one of the most common throughout the world’s vineyards.

Impact: Attacks the green parts of the vine and results in loss of leaves and consequently a lack of photosynthesis and unripe grapes. Bud break and shoot growth can be stunted. The early stages of the disease are indicated by an “oil spot” on the leaf, which progresses to patches of a white, fungal-type growth.

Remedy: Copper-based sprays can be used as a method of prevention, but need to be applied regularly, while fungicides can help kill the disease. Planting resistant rootstocks is another option. More natural methods include canopy management and bunch thinning to ensure airflow to reduce humidity and deter growth.

Fun fact: While downy mildew has penetrated nearly all of the world’s vineyards, some region’s remain free from it thanks to their low spring and summer rainfall. These places include Afghanistan, northern Chile, Egypt and Western Australia.

(Black) frost

Type: Climactic

What is it: Black frost is a type of frost that occurs with no white surface and are more severe, and is the level of frost that causes the most damage to vineyards.

Origin and cause: Black frost occurs when the humidity is too low for ‘white’ frost to form, but when the temperature is below zero, causing plant tissues to freeze and die. White frost tends to be less cold, partly because of the latent heat that the freezing of the water releases, reducing the temperature drop.

Impact: Can kill fruit and shoots, resulting in widespread loss of crops in the worst cases.

Remedy: Frost protection is available but expensive. Planting vines on south facing slopes, or north depending on which hemisphere you are in, can help. Other methods include high trellising systems, so that grapes avoid the coldest of the air closest to the ground, but this offers only limited protection. More extreme measure include wind machines, which circulate the cold air settling across a vineyard, mixing it with warmer air, while fans can also be used to blow hot air across a vineyard. Sprinkler irrigation can also be used to encase buds within a protective layer of ice to prevent tissue damage.

Fun fact: France witnessed extreme frost in 2016, with spring frosts hitting much of Champagne, Burgundy and the Loire Valley. Extreme first, along with an outbreak of mildew and hail, resulted in one of the smallest harvests in France for 30 years. 

Botrytis cinerea (grey rot)

Type: Fungal

What is it: Botrytis cinerea is the fungus that causes botrytis, which in its benevolent form is known as noble rot, responsible for some of the world’s best sweet wines. This occurs when grapes that are already fully ripe and healthy are infected. When grapes are infected before they are fully ripe, the disease takes a more destructive form, known as grey rot.

Origin and cause: Grapes are infected through minute pores in the grape, known as stoma, or broken skin, most commonly during humid weather, with grey rot more prevalent in vineyards that experience high rainfall with warm weather.

Impact: Spreads through the flesh of the grape and causes the skin to break down and the grape to rot. Wines produced from this fruit typically lack acidity, take on off-flavours, and lack colour and are generally unpleasant. Significantly reduces yields.

Remedy: The most common control measure for grey rot has been the use of fungicides, specifically copper sulphate, which can prevent its development if applied repeatedly from flowering to harvest. However as the Oxford Companion to Wine notes, “the fungus seems to be waging war against the chemist”, making natural methods an increasingly necessary back up. Bunch thinning and canopy management can reduce humidity and the growth of botrytis, while a reduction in the use of nitrogen-rich fertilisers can help to produce grapes with thicker skins, making them more resistant to the disease. Some producers that prefer to use more natural methods will accept some level of grey rot, and yield reduction, choosing instead to sort grapes, cutting out the affected berries. 

Fun fact: The word ‘botrytis’ originates from the Greek ‘botrus’, meaning cluster of grapes’.

Phylloxera

Type: Insect

What is it: Phylloxera is a microscopic aphid that eats the roots of grapevines.

Origin and cause:  Native to north America, phylloxera is believed to have been inadvertently introduced to Europe by Victorian botanists who had collected specimens of American vines in the 1850s and brought them back to the continent.

Impact: In the late 19th century a Phylloxera epidemic erupted throughout Europe coming close to killing pretty much every vine on the continent and all of its grape varieties.

Remedy: There are currently no effective chemical treatments or biological controls for phylloxera, with the only option to eradicate the disease to remove and burn the infected vines. Replanting on resistant rootstocks is the best course of action to prevent phylloxera. By the end of the 19th century the practice of grafting native American rootstocks onto European vines was widespread. While resistance to the bug has increased thanks to grafting, it remains a threat.

Fun fact:  At the height of its spread, some French winemakers buried a live toad under each of their vines, believing it would draw out the poison.