8. Hazards, Pests and Diseases Flashcards
(116 cards)
1
Q
What are the risks of drought?
A
Because the vine closes the stomata to limit water loss, photosynthesis is reduced or stopped. If the situation is prolonged;
* The growth of the plant is impaired
* Grape size is reduced and ripening slows down.
* This results in potentially unripe grapes and lower yields.
* If prolonged, vines will lose their leaves and die.
2
Q
What are the management options to minimise the effects of drought?
A
- Irrigation systems where allowed; some EU regions allow irrigation in emergency situations
- Drought tolerant rootstocks; eg rootstocks from V. rupestris and V. berlandieri parentage (e.g. 110R and 140R).
- Drought tolerant grape varieties; eg Grenache
3
Q
What are the risks of excess rainfall and water?
A
- Too much vegetative growth; competition with grape ripening, fruit is too shaded (both leads to less ripeness).
- Rainfall leads to high humidity, therefore fungal disease pressure
- Waterlogging reduces the amount of oxygen available to the roots, slowing down growth and eventually killing the vine.
- Waterlogging can also lead to the compaction of the soils, making them difficult to work, and uncontrolled water run-off.
4
Q
How can a grower mitigate the effects of excess rainfall?
A
If there is excess rainfall on a regular basis, then the planning of the vineyard must ensure that this issue is addressed by, where possible, planting on a slope or on free-draining soil to improve drainage, or by the construction of a drainage system
5
Q
What are the risks of excess rainfall during pollination and fruit set?
A
Can lead to millerandage or coulure, reducing the size of the crop and potentially also lowering quality.
6
Q
Why is excess rainfall in the summer a hazard?
A
Rain during the summer during grape ripening can reduce the rate of ripening of fruit.
Mild water deficiency before véraison reduces the growth of shoot tips, allowing the grapes to ripen more satisfactorily.
7
Q
Why is excess water just before harvest a hazard?
A
Heavy rainfall close to vintage can lead to the grapes being swollen with water (reducing the concentration of the must and with it the quality of wine) and to grapes splitting, leading to grey rot. It also makes working the harvest very difficult. Examples include difficulty using mechanical harvesters or pickers accessing the vineyard if on clay soils.
8
Q
How can a grower mitigate the effect of untimely rainfall?
A
- Choice of site (climate, slope etc).
- Condition of the soil
- Choice of whether or not to grass the land between rows of vines
- The adequacy of drainage
- Monitor weather forecasts to decide if early harvest (with potentially less ripe fruit) or taking the chance on the weather improving (risk of losing part or all of the crop).
9
Q
What parts of the vine are the most at risk of winter freeze?
A
- The graft if it is above the surface of the ground
- Canes and cordons
10
Q
What regions are most at risk of winter freeze?
A
Areas most affected are those with a strongly continental climate such as parts of Canada, Washington State and China.
11
Q
At what temperature are vines at risk of damage by winter freeze?
A
-20°C and below can damage or kill vines.
12
Q
What sites are desirable in areas with very cold winters and reduce the risk of winter freeze?
A
- Hillside sites can be up to 5°C warmer than the valley floor.
- Vineyards near large or deep bodies of water benefit from a moderating effect.
- Vines should be planted where snow settles most thickly, as a deep layer of snow can provide insulation for the vine.
13
Q
Give some examples of species and varieties that can be chosen for areas at risk of winter freeze?
A
- Some varieties of vitis vinifera are more resilient against winter freeze than others, e.g. Cabernet Franc or Riesling.
- Some American and Mongolian vine species (V. amurensis) are extremely winter hardy, as are hybrids that have these species as a parent. For example, the Concord variety, an American hybrid, can withstand temperatures down to nearly −30°C / −22°F.
14
Q
How can a grower protect vines from winter freeze?
A
** Building up soil** around the vine graft (often called ‘hilling up’) is common practice in regions with cold winters, as the soil helps to insulate the vine.
* Burying vines is another approach in the most extreme climates. However, this is
a very costly approach as it requires a lot of labour every year, with some Chinese regions seeing this as one of the costliest operations in the vineyard.
15
Q
What are a growers options to mitigate the effects of very cold winters (winter freeze?
A
- Site selection
- Choice of grape variety
- Protecting the vines (building up or burying).
16
Q
How do frosts occur?
A
Frosts occur when cold air below 0°C collects at ground level, freezing water in the vine’s growing buds and shoots.
17
Q
Why are warm areas vulnerable to spring frosts?
A
Warmer areas are vulnerable because the vine begins to grow and will be damaged if there is a drop in temperature.
18
Q
How does the vine react if young buds and shoots are killed?
A
the vine puts out more shoots from secondary buds; however, these shoots are less fruitful and will take longer to ripen, which can itself be a problem (more likely to be affected by rain or autumn frost).
19
Q
What are the two types of frosts?
A
- Advective
- Radiative
20
Q
What are advective frosts?
A
Advective frosts are caused by large volumes of cold air moving in from very cold areas.
21
Q
What are radiative frosts?
A
Radiative frosts are the result of heat being lost on still, cool nights.
The earth is heated by the sun during the day and releases it during the night. The amount of heat lost depends on the level of cloud cover.
Windless nights will allow a layer of freezing cold air to develop just above the surface of the soil.
As cold air is denser than warm air, this freezing cold air will collect in valley bottoms.
22
Q
How can a grower reduce the risk of frosts?
A
- In site selection, care must be taken to avoid frost pockets and to choose hillside sites where cold air can drain away.
- Delaying pruning postpones budburst into warmer months. If buds at the end of canes get frosted, they can be removed.
- Choosing a variety that buds late, such as Riesling, can help.
- Vines trained high off the ground offer more protection, as the coldest air is near the
ground.
Having bare soil between the vines (rather than a cover crop) absorbs more heat during the day and radiates this heat during the night.
23
Q
What are a grower’s options when a frost threatens a vineyard?
A
- Water sprinkers (aspersion)
- Wind machines
- Oil or propane gas burning heaters and wax candles
24
Q
What is aspersion and how does it work?
A
Water sprinklers to combat frost
* As water freezes around the parts of the plant, it releases latent heat, protecting the plant. The system must be kept on until the temperature rises.
25
What is the only method that can combat against advective frosts?
Water sprinklers (aspersion)
26
How do wind machines combat frosts?
These large fans, which are 4–7 m high, pull warmer air from above down to ground level, thereby raising the temperature. These are effective where there is an inversion layer, a warm zone of air 10 m above the ground (+3–5°C / 5–9°F).
27
What are the costs associated with aspersion?
The costs are for the equipment and for the water. The running costs can be much lower than wind machines or heaters, although not if the cost of water is high.
28
What are the costs associated with wind machines?
One study has found that the investment in wind machines is warranted if there is a chance of a damaging radiation frost once every five years.
The initial investment is considerable.
Helicopters can be used to create the same effect. They are expensive, but may be worth it if the risk is severe but short term.
29
What are smudge pots and bougies?
Smudge pots; Oil or propane gas burning heaters
Bougies; Wax candles
Can be placed in vineyards and lit when there is a risk of frost.
30
What are the disadvantages of smudge pots and bougies?
The high cost of fuel and labour, low heating efficiency and contribution to air pollution.
## Footnote
Smudge pots; oil or propane gas burner heaters
Bougies; wax candles
31
What are the strategies to minimise the risk or damage of hail?
* Rockets
* Netting
* Owning several plots
* Crop insurance
32
How do rockets prevent hail?
Rockets may be fired into thunderclouds, seeding them with silver iodide to cause rainfall rather than hail.
33
What needs to be considered before implementing netting for hail protection?
As netting creates some shading, this solution can only be used where there are high sunlight levels. It is therefore more appropriate in Argentina than in Burgundy.
34
How do grapes become sunburnt?
In prolonged warm, sunny weather, grape transpiration is much more limited, and therefore less effective, than leaf transpiration. As a result, grapes can reach higher temperatures than the leaves and become burnt.
35
How does sunburnt affect grapes?
* Sunburn leads to scars on the skin of the grape and, in extreme cases, to the eventual death of the grapes.
* Sunburn has a negative impact on grape quality. Browning of the grape, a bitter taste, and increased susceptibility to rot (due to skin damage).
36
How can growers reduce the risk of sunburnt grapes?
* In designing new vineyards, row orientation and aspect can reduce the impact of the hottest afternoon sun.
* The amount of direct sun exposure can be adjusted through canopy management techniques. Growers may choose to partially shade the fruit zone in hot regions.
* If a heatwave is forecast, additional irrigation, where allowed, may be applied to reduce water stress and, hence, the chance of sunburn.
* Special agricultural sunscreen spray can be applied or the vines can be shaded with a cloth or net.
37
What factors make a vineyard more susceptible to fire damage?
Vineyards that are near woodland, pastures or other crops may be more at risk as they provide fuel for the fire. Cover crops and organic mulches can also provide fuel
38
How can growers prepare their property if a fire threatens?
* Installing fire detectors and sprinklers
* Installing and maintaining a water tank
* Providing employee training for action in the event of an emergency.
39
How does smoke affect grapes and wine?
Aroma compounds in smoke can be absorbed by the grapes. Once in the grape these compounds often bind with sugars and form aroma-less precursors.
Similar to other aroma precursors in the grapes, these compounds then only become aromatic through the fermentation process. The strength of the aroma can increase during the ageing of wine and during bottle ageing as further aroma precursors break down and become aromatic.
40
What are the management options against smoke taint?
* Affected musts can be tested analytically and/or by micro-vinifications (to release the smoke aromas) in the days leading up to harvest to establish the extent of the problem.
* As it is thought that the smoke aroma precursors are present on the inside of the skin of grapes, how the grapes are handled can reduce the effect of smoke taint. Hand harvesting, gentle or whole bunch pressing, lower fermentation temperatures and reduced maceration times can reduce the uptake of the compounds.
* Flash détente and reverse osmosis can also help, but will not remove the taint completely.
* Blending with unaffected wine
41
What are the most important climactic hazards?
* Drought
* Excess water
* Untimely rainfall
* Hail
* Frost
* Winter Freeze
* Sunburn
* Fire
* Smoke taint
42
How does phylloxera affect the vine?
This aphid-like insect feeds and lays eggs on the roots of grapevines.
Phylloxera harm vines by damaging the roots, reducing the uptake of nutrients and water. The damaged roots are also vulnerable to additional attack by bacteria and fungi. These attacks lead first to a weakening of plants (reducing growth and yield) and then to the death of the vines.
43
How does phylloxera spread?
The insects can spread through crawling but are most commonly transported by humans, for example, on the roots of young vines, in soil, on equipment such as leaf trimmers and harvesters and by irrigation water
44
What are the symptoms of a phylloxera infestation in the vineyard?
* Vines die of drought in patches that increase in size year by year
* Vine roots are covered with the insects surrounded by yellow eggs
* Swellings on older roots
* Pale green leaf galls on the under-surface of the leaves
* Slow, stunted shoot growth and leaf yellowing appears in around three years, the plant dies after around five years.
45
What soils inhibits phylloxera?
Sandy soils
46
Which American vine species provide the most protection against phylloxera?
V. berlandieri, V. riparia and V. rupestris
47
How are American vine species resistant to phylloxera?
These species form hard, corky layers that surround the eggs, sealing the wounds and preventing invasion by bacteria or fungi.
48
Why were rootstock hybrids created? Why not use a single American variety for grafts?
* Grafting onto the rootstocks of single American
varieties caused problems in the typically calcareous soils of Europe as these varieties have little lime tolerance.
* The vines suffered from chlorosis turning the leaves yellow, halting photosynthesis and reducing yields and quality.
* The solution was to create rootstock hybrids between the various American species in order to balance the level of protection to phylloxera and resistance to lime in the soil.
* Rootstock hybrids can now deal with several issues (phylloxera, nematodes, extremes of soil pH, water stress, salinity – and control the vigour of the vine.)
49
What are nematodes, and how do they damage the vine?
Nematodes, microscopic worms, are very common in soils, but are usually too small to be seen by the naked eye.
Some cause damage by feeding off vine roots, significantly reducing yields and vigour. They can cause slow, gradual decline. Others transmit viral diseases.
50
What are the two most commonly occurring nematodes?
Root-knot nematode
Dagger nematode
51
Can nematodes be eliminated from the soil?
No, only managed.
52
What are the management options against nematodes?
* Fumigate the soil. Chemicals used to be used, but these are now banned in most regions. Another method is to plough in a cover crop of mustard plant, which contains compounds that work as biofumigants, killing nematodes.
* For most, the best solution is the use of nematode-resistant rootstocks having ensured that plants bought from nurseries have been heat-treated to kill nematodes. Examples of nematode-resistant rootstocks include Ramsey and Dog Ridge (both Vitis champini).
53
How do grape moths damage the vine?
A number of different moths do damage to vines by feeding on flowers and grapes. Many species have several generations per season, attacking flowers in spring and grapes later in the year. The wounds created are then vulnerable to further attack from bacteria and fungi, including botrytis, and significant crop losses have been reported.
54
What are the most common types of grape moths?
The most common types are the light brown apple moth in Australia, the European grapevine moth in southern Europe and the grape berry moth in central and eastern North America.
55
How can grape moths be controlled?
* Biological controls include the use of:
– the bacterium Bacillus thuringiensis (which produces substances that are toxic to the moths)
– use of pheromone capsules to disrupt mating (‘sexual confusion’)
– natural predators (parasitic wasps, green lacewings, some spider species).
* Insecticides can be used.
56
How do spider mites affect the vine?
They feed on the surface cells of leaves. This leads to discoloration of the leaves, a reduction in photosynthesis, delayed ripening and a reduction in yields.
Spider mites thrive in dusty conditions and are most damaging when vines are already water stressed.
57
What kind of spider mite affects vineyards in California?
The pacific spider mite
58
What spider mites affect vineyards in Europe?
The red spider mite
Two forms of yellow spider mites
59
How can spider mites be managed?
* Make the environment inhospitable by the use of water sprinklers and/or cover crops or mulches to reduce dust.
* Encourage predatory mites (by planting host species) that feed on spider mites.
* General pesticides may kill beneficial predatory mites. Specific sprays can be applied to
kill only the mites that are problematic in the vineyard, but this can add to costs.
60
How do lady birds affect vines?
They can cause taints to the wine if they are in amongst harvested grapes.
61
What are the management options against birds?
* The cost of total netting can be justified in high value areas (for example, Mornington Peninsula in Australia) or where birds are a major threat.
* Other measures such as bird scarers or noises can be used, but must be rotated regularly to avoid the birds getting used to them. Falcons are sometimes used to deter unwanted bird visitors.
62
What are the main pests that pose a risk to vineyards?
* Phylloxera
* Nematodes
* Grape moths
* Spider mites
* Leaf hoppers
* Lady birds
* Birds
* Mammals
63
What fungus causes powdery mildew?
Grapevine powdery mildew is caused by the fungus *Erysiphe necator* which is also commonly called *Oidium tuckeri.*
64
How does powdery mildew affect vines?
Powdery mildew overwinters in buds and on canes. It then attacks young, green parts of the vine. Affected parts of the vine show as dull grey patches and become black patches as they advance. Patches can damage young shoots, inflorescences and grapes, reducing yield.
Grapes can also split at véraison and become targets for other infections
65
What fungal disease are these grapes affected by?
Powdery mildew
66
What are the optimum conditions for powdery mildew?
The growth rate is determined by temperature, with an optimum temperature around 25°C, and it thrives in shady conditions.
Unlike other mildews it does not require high humidity and so can spread in relatively dry conditions, especially in dense, shady canopies.
67
What are the management options against powdery mildew?
* The preferred approach is to keep an open canopy to reduce shade and the density of leaves.
* Applications of sulfur help to prevent and treat the disease. Growers will spray the vines from a couple of weeks after budburst and up to véraison. It is important to spray early in the season as the disease is easier to prevent than to contain if it gets established.
* Systemic fungicides can be effective and, as they penetrate the green tissue of the vine, are not washed off by rain. However, the fungus can become resistant to some fungicides, so only a limited number of applications can be made in one year.
* Experiments with UV light seem to be effective.
* New organic product called HML32 also in trials.
68
What fungus causes downey mildew?
Downy mildew is caused by Peronospora, a water mould that lives within vine tissue, not on the surface.
69
How does downey mildew affect the vine?
It attacks green parts of the plant, especially young leaves and flowers, reducing yields by defoliating the vine. Grapes can also be affected, but this is less important than the threat of defoliation.
70
What are the optimum conditions for downey mildew?
It needs rainfall and warm temperatures (20°C / 68°F) to spread. High-risk periods are warm springs and stormy but warm summers.
71
What are the symptoms of a vine affected by downey mildew?
The symptoms are yellow, circular ‘oil spots’ and then white, downy fungal growth on the underside of leaves.
72
What fungal disease is this vine affected by?
Downey mildew
73
What are the management options against downey mildew?
* Traditionally, sprays made from copper salts have been used to prevent the spread of downy mildew. So-called Bordeaux mixture, a combination of copper sulfate and lime, became the standard treatment from the 1880s. Protection from these copper sprays only lasts until 20 mm of rain has fallen. This is the only option currently available to organic growers.
* Other fungicides can also be used.
* Good drainage and an open canopy that dries quickly are helpful to avoid the fungus developing and spreading.
74
What is a Bordeaux Mixture?
A combination of copper sulfate and lime used against fungal diseases.
75
What is grey rot?
Botrytis Cinera
76
What is laccase?
Oxidative enzymes produced by botrytis rot
77
What makes grapes more susceptible to grey rot?
Grapes are vulnerable if there are any points of entry (e.g. grapes having rubbed against each other in tight bunches or been punctured by birds/insects), leading to whole bunches being attacked.
Varieties with tight bunches or thin skins are most at risk: Semillon, Sauvignon Blanc, Pinot Noir and many others
78
What happens when a vine's flowers are affected by grey rot?
If the flowers are affected, the fungus can stay dormant in the grape and re-emerge after véraison.
79
What are the management options against grey rot?
* Selecting grape varieties that have small grapes with thick skins and therefore high levels of resistance and protecting the grapes against other pests (which could split the skin of the grape) are the most important options.
* Keeping an open canopy and removing the leaves around bunches can also reduce the spread of grey rot.
* Traditional sulfur and copper sprays are ineffective, but other fungicides can be used. They should be applied at key points in the season. However, fungicides quickly become ineffective if the fungus develops resistance.
* Attention has therefore turned to using antagonistic bacteria; for example, Bacillus subtilis and other forms of biological control.
80
At what points in the season are fungicides most effective?
When flowering is nearly complete, at the end of grape formation, at bunch closure (when the grapes in a bunch get large enough so that they touch each other) and véraison.
81
What is Eutypa Dieback?
Eutypa dieback (also called Dead arm) is a fungal trunk disease that leads to rotten wood in vines and can affect whole vineyards.
It reduces yields significantly and kills vines over a ten- year period if not tackled.
82
How does a vine become infected by Eutypa Dieback?
Spores are spread by wind over long distances. Infection occurs through pruning wounds in moderate temperatures and especially during rain.
83
What are the symptoms of Eutypa Dieback?
The effects of the fungus are evident in spring with affected vines displaying stunted shoot growth and yellow leaves
84
Name three grape varieties particularly susceptible to Eutypa Dieback.
Grenache, Cabernet Sauvignon and Sauvignon Blanc
85
Name three areas in which Eutypa Dieback is particularly prevalent.
South Australia
South-West France
Parts of California
86
What are the management options against Eutypa Dieback?
* Eutypa dieback is difficult to control, although pruning late and applying fungicide to pruning wounds can be effective.
* Affected trunks can be cut back 5–10 cm beyond the visible symptoms and treated with fungicide. Dead wood must be burnt to avoid spores spreading.
* Some biological controls (e.g. Bacillus subtilis) may be effective.
* If a plant is badly affected, the options are either to retrain from a sucker (a shoot that grows from the base of the vine) left on the trunk (which causes a loss of yield for two years) or removing the vine and replanting it.
87
What is phomopsis?
Phomopsis is a fungal disease that causes a reduction in yield.
88
What are the symptoms of phomopsis cane and leaf spot?
Infected canes whiten and break off easily. Shoots growing from these canes develop brown cracks at their bases. Leaves are also affected showing yellow/brown spots.
89
What are the favourable conditions for phomopsis cane and leaf spot?
It is particularly prevalent in years with cool and wet springs followed by humidity and moderate temperatures.
90
What are the management options against phomopsis cane and leaf spot?
* Fungicides should be applied three weeks after budburst and then every two weeks if wet conditions continue.
* Diseased and dead wood should be removed during pruning and the removed wood (prunings) then burnt or buried.
* Canopy management techniques that improve air flow within the canopy may also reduce risk of the disease.
91
What is ESCA and how does it occur?
Esca is a complex fungal disease caused by a group of organisms particularly prevalent in warmer and drier climates. It typically enters the vine through pruning wounds.
92
What are the symptoms of ESCA?
Symptoms include tiger-striping of the leaves and spotting inside the wood.
93
What are the management options against ECSA?
As there are no chemical controls, most attention has been paid to **prevention** of the disease: sourcing disease-free stock, trying new, less detrimental, pruning techniques, not pruning in the rain, removing prunings promptly from the vineyard and disinfecting pruning wounds.
Research is continuing into using biological agents such as Bacillus subtilis.
94
# 10
What are the most common fungal diseases that affect vines?
1. Powdery mildew
2. Downey mildew
3. Grey rot
4. Eutypa dieback
5. Phomopsis cane and leaf spot
6. ESCA
7. Black rot
8. Black foot disease
9. Bot Canker
10. Anthracnose
95
What are the main bacterial diseases that affect vines?
* Pierce's disease
* Grapevine yellows
* Bacterial blight
* Crown gall
96
How does Pierce's disease attack vines?
The bacterium lives in the sap channels of vines, which it clogs, leading to grape shrivelling, dropping leaves and the death of the vine between one and five years.
97
What symptoms does the vine show when infected by Pierce's disease?
The exact symptoms are unclear and so vines must be tested in a laboratory to ascertain whether they are infected.
98
What vector is known to spread Pierce's disease?
The sharpshooter insect
* The glassy-winged sharpshooter is responsible for the quick spreading of the disease since the 1980s.
* Other species include the blue-green sharpshooter, green sharpshooter and red-headed sharpshooter.
99
Name two grape varieties particularly vulnerable to Pierce's disease.
Chardonnay and Pinot Noir
100
What are the management options against Pierce's disease?
**There is no chemical control for the bacterium.**
* Control is by reducing the number of the vector. For example, removing vines close to rivers has been effective as riverbanks can be a habitat for one of the vectors, the blue- green sharpshooter. Some chemical insecticides can also be used. Introducing a species of wasp that feeds on the eggs of sharpshooters has also been effective.
* Strict quarantine rules for the movement of plants have sought to prevent the further spread of the disease.
* For the future, work is being done on developing Pierce’s Disease-resistant vines.
101
How are grapevine yellows spread?
The disease is spread by vectors, which include leafhoppers, and by nurseries selling untreated, diseased stock.
102
What is the most common type of grapevine yellows?
Flavescence dorée
103
What are the symptoms of flavescence dorée?
Symptoms include delayed budburst, a drooping posture because the new shoots fail to become woody, and the canopy turning yellow (in white varieties) and red (in black varieties). In some strains, the vine dies as the disease progresses, in others it can recover after an attack.
104
In which countries is flavescence dorée present?
France, Germany, southern Europe, New York State and, in a different form, in Australia.
105
Name two examples of grape varieties most vulnerable to grapevine yellows?
Chardonnay and Riesling
106
What is flavescence dorée?
The most common type of grapevine yellows
107
What are the management options against grapevine yellows?
* There is no control against the bacteria that cause for grapevine yellows.
* The focus is on controlling the vector. Leafhopper populations can be reduced by insecticides, and the plants that host the hoppers, including cover crops, should be removed.
* Best practice in the nursery is to bathe the pruning wood in hot water to kill the disease.
108
What are the main two groups of viruses that affect vines?
Fanleaf virus
Leafroll virus
109
What are the symptoms of fanleaf virus?
Early shoot growth is stunted, canes can grow in distorted ways and leaves are very pale, malformed and can look like a fan. The effects vary widely from little effect to losing most of the crop in susceptible varieties such as Cabernet Sauvignon.
110
How are fanleaf viruses spread?
The disease was spread enormously by the move to grafted vines following phylloxera and the inadvertent use of infected plant material. Otherwise, the disease is spread slowly by the dagger nematode.
111
What are the management options against fanleaf viruses?
* There is no cure for the disease and, eventually, affected vines will have to be removed and replaced, adding to cost.
* Before vineyards are replanted, soil tests should be done to check for the presence of dagger nematodes and only virus-tested, clean planting material should be used.
112
How are leafroll viruses spread?
The virus was spread by grafting and by mealy bugs
113
In which regions are mealy bugs a key pest?
South Africa, the Mediterranean, Argentina and some parts of California
114
How do leafroll viruses affect vines?
While the condition does not kill vines, it can reduce yield by up to half and affect quality negatively. Leafroll virus slows down the growth of roots and shoots. Surviving fruit may take several additional weeks to ripen and have more acidity, less colour and lower sugar levels. The overall health of the vine is also affected as it stores less carbohydrates.
115
What is the key symptom of leafroll virus?
The typical downward rolling of the vine’s leaves usually occurs in autumn. The leaves change colour in the autumn to red for black grape varieties and to yellow for white grape varieties.
As the symptoms are not always clear, vines have to be tested in a laboratory. Some vines and rootstocks carry the virus without showing symptoms.
116
What are the management options against leafroll virus?
* There is no cure for leafroll virus and therefore the only solution is to remove unproductive vines and replant with virus-free stock.
* Nurseries can screen vines for virus infections.
* Mealy bugs favour humid environments and therefore open canopies help to reduce the
pest. Control by spraying is difficult because of the mealy bug’s waxy coating. Steps can be taken to encourage the mealy bug’s natural predators: ladybugs, lacewings and others.
