It is impossible to overstate the devastation that grapevine diseases can cause. In 2023, Italy reported an overall decrease in wine production of 12% on the previous year. This was in part because of the extreme weather events experienced, but also thanks to the impact of downy mildew in the central and southern regions. The damage to yields caused by the fungus means that after nine years of holding the crown, Italy is no longer the world’s biggest wine producer.
The world of disease management is a rapidly evolving one. Research into disease detection and protection measures is ongoing. Climate change also has a role to play in throwing new challenges at winemakers worldwide. The prevalence of downy mildew in Italy last season was caused by notably increased rainfall, for example.
This month, I take a look at the current state of disease detection as it relates to UK vineyards, including emerging threats and the cutting-edge technologies being employed to protect our crops.
Key pest and disease events in 2023
Despite being a bumper year for yield overall, July last year was a disappointing month with high rainfall and low light levels. This generated an increased risk from fungal diseases like grey mould and downy mildew. Traditional fungicides were limited in their effectiveness under these circumstances because they have a harvest interval of three to five weeks. Good canopy management was therefore vital to increase air movement and to expose the bunches to optimise the penetration of the limited number of spray applications where fungicides were used.
There were also considerably more reports of Spotted Wing Drosophilia than in previous years. This has widely been attributed to lower levels of acidity in the grapes, particularly in black varieties. Stephen Skelton’s Harvest Report 2023 for Wine GB also notes that the “increased cropping area of red varieties such as Pinot Noir Precoce (Blauer Fruhburgunder) which was 30ha in 2017 and now (2022) stands at nearer to 100ha.” Some growers reported losing the full crop of some varieties due to SWD. Others were forced to pick early as sour rot set in.
Know your enemy
Downy mildew (Plasmopara viticola) – a fungal disease which can overwinter from one season to the next. The fungus thrives in damp conditions and regular heavy rain causes it to spread quickly. Early detection and treatment are vital to prevent the rapid development of downy mildew throughout the canopy. The primary infection manifests as a yellow oily spot on the top of the leaf. The secondary infection shows as white down on the corresponding underside. This infection causes shoots to brown and unprotected fruit can show white down and rachis. The berries will eventually turn necrotic.
Grapevine trunk disease (GTD) – The name for a family of fungal diseases that can devastate a vineyard through decreasing vine quality and lowering yields. The fungi enter the vine through open wounds, normally pruning cuts. Pruning in dry conditions when spores are less likely to have been dispersed can help, as can late pruning when rising sap can help to displace spores on cuts.
Grey mould (Botrytis cinerea) – The most virulent grapevine disease in the world. It tends to impact varieties with tight bunches and thick canopies that restrict airflow. Infected bunches get a layer of grey brown fungus which causes them to decay and drop. Leaves can also show signs of infection, with dull green patches that will eventually turn necrotic. New infections are primarily caused by the transfer of spores from old plant material, so winter pruning is particularly important to remove diseased material, which then needs to be properly disposed of.
Insect pests – The likes of leafhoppers and cochylis vine moth can bring two waves of damage to vines. As well as directly impacting the plant, they can also transmit the pathogens which cause disease. For example, the leafhopper may nibble on leaves, leaving areas which cannot photosynthesise, eventually causing the leaf to drop. But they are also known vectors for infections like Flavescence dorée disease. Cochylis promotes the development of Botrytis.
Powdery mildew (Erysiphe necator) – A fungal disease that infects green tissue and has a pale grey, powdery appearance. It can lead to crop loss, reduced fruit quality and off-flavours in wine. The severity of the infection depends upon the weather conditions, levels of inoculum carried from the previous year and the control measures taken early in the season.
Advances in disease detection technologies
Technology is being applied to the fight against disease. Early detection and precision management of vineyard health are helping to keep losses to disease at a minimum around the world.
Researchers in Southern California have used an airborne instrument that was developed at NASA’s Jet Propulsion Laboratory for remote sensing to pick up signs of disease before the symptoms are detectable to the human eye.
Their study looked at a viral disease named GLRaV-3 – grapevine leafroll-associated virus complex 3. It is spread by insects and results in reduced yields and sour fruit. The disease is estimated to cost the US wine industry around $3b in losses and damage each year. While GLRaV-3 is thought to occur in all wine-growing regions of the world, it is currently assessed as a moderately low risk on the UK Plant Health Risk Register because the UK distribution is very uncertain.
Previously, detection required intensive examination of individual vines and molecular testing. The Airborne Visible/InfraRed Imaging Spectrometer is used in combination with machine learning to help identify early signs of infection. The Spectrometer’s optical sensor records the interaction of sunlight with chemical bonds, meaning that changes in the way sunlight interacts with the plant tissue could be detected well before visible signs of infection like discoloured foliage occur.
The researchers were able to identify infected vines both before and after they displayed symptoms of the disease. The best-performing models were accurate up to 87%. This early detection is thought to give growers up to a year’s warning to act. This sort of technology is likely to be trained to recognise the signs of other diseases in the future to give early warning of infection.
Genetic approaches to disease resistance
Also in the United States, researchers have been looking at how genetic mapping could identify genes which could be introduced to grapevines to enhance their disease resistance. A Californian study into Xylella fastidiosa, which can infect many crops including grapevines, was begun after the research team found a wild grape species, Vitis arizonica, exhibits a rare natural resistance to Xylella.
It is worth noting that Xylella has not yet been reported in the UK, and genetically modified foods for sale in this country must have a Food Standards Agency safety assessment. However, the exploitation of the genetic variability of individual plants and varieties has widely been employed in traditional plant breeding both in the UK and abroad. Now progress in scientific understanding is helping to move the breeding of desirable traits quicker than ever before.
The availability of sequenced genomes has improved knowledge of virus resistance in grapevines. For example, the Riesling clone 29 was shown to have a natural resistance to GFLV strain 13. This resistance has been associated with a single recessive factor in chromosome 1, which happily means that “it is stable through transmission to offspring and easy to use in breeding programs, despite being recessive.”
It seems likely that in years to come, more disease-resistant strains of popular varieties will begin to appear. There is an economic imperative to meet this need, particularly as some diseases are becoming more virulent where climate change is affecting weather patterns in wine-producing regions.
Integrated pest management strategies
The overall approach to managing diseases can be categorized as an Integrated Pest Management (IPM) strategy. This might include any aspect of the holistic care that is taken to fight disease – right through from careful site selection when vines are planted, to minimise risks like high humidity for example, to the mechanical processes of removing diseased tissue when disease has already struck and its impact needs to be contained.
A widely held aspiration amongst grape growers is to reduce the number and amount of chemical treatments being used on the vines. Many approaches can be taken to do this within the IPM. Some examples include:
Growing cover crops – As well as offering additional nutrition and protecting the soil, cover crops can be an effective way of increasing biodiversity which encourages pest predators to thrive.
Mechanical weeding – Instead of using herbicide to control weeds, mechanical methods can be employed during the growing season. At Danebury Ridge in Essex, they do this as well as allowing a flock of sheep to graze in the winter to provide weed control without soil compaction or risk to the vines.
Canopy management – Leaf trimming, thinning and pruning out superfluous shoots can moderate the impact of infection or pest colonisation. Good management opens up the interior of the canopy to encourage the circulation of air and increase the exposure of the remaining leaves and any fruit to the light.
Vermicast
Some vineyards have been making use of vermicast to reduce the amount of chemical treatment required by their vines. Vermicast is created by earthworms and is a microbe-rich product that can improve vines’ resistance to pests and disease as well as improve their uptake of nutrients and minerals from soil. I talked to Luke Boxall, a worm farmer and owner of Soil Nurture to find out more.
“Vines are just like humans. The healthier they are, the more resistant they are to pests and diseases. A healthy vine has a strong interaction with the environment they are in. This can only be achieved through a healthy microbiome, full of beneficial microbes. Earthworms are key instruments in providing these beneficial microbes and have been doing so for over 200 million years.
“By consuming organic matter, earthworms produce vermicast which has a rich and diverse population of beneficial microbes. Studies conducted on various plants, including vines, show that vermicast can help these plants become more resistant to pests and diseases due to its microbial content. In the UK, we have seen how vines with vermicast are more resistant to mildews and botrytis, and on the occasions where the vines have been overwhelmed by the disease, the healthier vines with vermicast have recovered faster. This means that fewer chemicals have been used in the vineyard.
“By adding vermicast to a vineyard, the beneficial microbes combined with high levels of fulvic and humic acid, plant hormones and plant-growth-regulating substances (including auxins, gibberellins, cytokinins, ethylene, and abscisic acid), can help a vineyard reduce the amount of chemical inputs required. By reducing chemical inputs, the vineyard is able to increase the interaction of the vine with its environment, increase its local biodiversity and improve the quality of the wine produced, ensuring the complex and unique terroir of the vineyard is brought out in every glass of wine.”
Emerging threats and future challenges
New challenges are continually emerging to test the vineyard manager. These often come in the form of new insect pests, which may be spread over long distances through the movement of infested plant material.
DEFRA issued a UK pest alert for Flavescence dorée disease two years ago since it was known in the EU. Considered to be one of the most destructive phytoplasma diseases for vines, it is spread by the American grapevine leafhopper, which is a vector for the Candidatus Phytoplasma vitis organism. While the insect is a foreign invader, the Phytoplasma strain itself is native to Europe, arising in wild plants. The disease can also be spread by propagation.
Flavescence dorée disease presents with colour change in leaves and downward rolling edges, shoots that fail to lignify and interrupted fruit set with berry drop and brown, shrivelled bunches. Infection can also kill young plants. There is no known genetic resistance to the disease, although evidence suggests that some varieties are more susceptible than others.
There is currently no effective treatment for Flavescence dorée disease, although management strategies to reduce the spread between vines and control the vector have met with some success on the continent.
Efforts have been made to automate the detection of Flavescence dorée through the use of unmanned aerial vehicle (UAV) remote sensing. Like the early detection of leafroll, this technology moves beyond imaging what is visible to the human eye and uses multispectral imagery as well. The spectral signatures of healthy and symptomatic plants were studied, producing a set of 20 variables computed from the UAV images.
Initial studies obtained the best results with red varieties, but deep learning has been employed to train the computer models to better accurately map the disease in white varieties too.
It is believed that spotted lanternfly has not yet reached the UK, but the native of northern China has been known as an invasive species in Korea since 2004 and the USA since 2014. This plant-hopper bug has had a significant impact on vineyards where large populations have affected fruit set in the following season as well as caused vine death.
While the insect can live on almost any tree and lay eggs on any object, making it a prime candidate for accidental importation, it is not yet known how the pest might respond to the UK climate. Our summers may be currently too cool for the insect to complete its life cycle here, although climate change may impact this situation in the future.
