The three main diseases that affect grapevines in the UK are;

1. Downy mildew on the leaves – caused by the fungus-like organism Plasmopara viticola
2. Powdery mildew on the leaves and fruit – caused by the fungal pathogen Erysiphe necator
3. Grey mould leading to fruit rot – caused by the fungal pathogen Botrytis cinerea

Grapevines are also rather susceptible to the root disease honey fungus.

Recently, RHS researchers have detected four viruses affecting grapevines in the UK - caused by Grapevine virus A; Grapevine rupestris stem pitting associated virus; Grapevine fleck virus; and Grapevine leafroll-associated virus 1

Typical symptoms for the three diseases are as follows;

Downy mildew

• Yellowish patches occur on the upper surfaces of leaves, often limited by the veins, corresponding to whitish patches of 'mould' growth on the lower surfaces

Powdery mildew

• White powdery fungal growth occurs on leaves, stems and fruits
• Sometimes growth of the powdery mildew fungus is sparse and the symptom is a grey or purplish discolouration of the leaves
• Fruit may become split or may drop prematurely if infected by powdery mildew
• Fruits damaged by powdery mildew are often then infected by the grey mould pathogen and rot, or become mummified

Grey mould

• Fruit becomes soft and brown, especially as it starts to ripen
• A fluffy grey mould may appear on the affected fruits, particularly when humidity is high

Symptoms of virus diseases

• Rugose wood complex - reduced vigour and delayed bud break. Swellings may develop, on grafted vines, above the graft union. In some cultivars, a thick, corky layer of bark forms. The stem may become marked by pits and grooves. May be transmitted by vegetative propagation, and also by mealybugs and scale insects' feeding activities
• Fleck disease - causes leaves to become wrinkled, twisted, flecked or show yellow spots or veins. The whole plant may be stunted. Transmitted by vegetative propagation only
• Leafroll disease - leaves become thick and brittle, leaf margins roll downwards, inter-veinal reddening on red-fruited cultivars, inter-veinal yellowing on white-fruited cultivars. May be transmitted by vegetative propagation, and also by mealybugs and scale insects' feeding activities
• Grapevine degeneration/decline - on leaves, vein banding, yellow mosaics or mottling may be seen, leaves may also develop malformed or fan-shaped, vines progressively decline producing low yields of poor-quality fruit

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

• Clean up and destroy fallen leaves to reduce the amount of the downy mildew pathogen surviving the winter
• Mulch plants each season to avoid drought stress, which predisposes them to powdery mildew infection, and make sure vines are adequately watered helping to prevent this problem
• Avoid overhead watering as this increases humidity around the foliage, which makes the plants more prone to powdery and downy mildews
• Keep vines under glass well ventilated
• Prune overcrowded shoots to improve air circulation around the foliage
• Consider heating the glasshouse in cold, dull weather to avoid excessive humidity

Resistance: A number of grape cultivars show some resistance to powdery mildew including the wine varieties 'Leon Millot', 'Orion', 'Seyval Blanc' and 'Triomphe d'Alsace' and the outdoor dessert variety 'Brant'.

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

There are no products available to amateur gardeners for the control of either downy mildew or grey mould.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: storing and disposing safely

Downy mildew: the downy mildew pathogen survives over winter as resting spores in dead, infected leaves. These initiate new infections in the spring. Subsequent infections occur from airborne spores produced on these initial infections. Wet conditions are required for infection to occur and the disease is very seldom seen on vines grown under glass. The pathogen is an obligate biotroph (it feeds on the living cells for extended periods without killing them, and cannot live on dead plant material). Vitis vinifera is susceptible; other Vitis spp. and related Parthenocissus spp. (Boston ivy, Virginia creeper) and Ampelopsis are less affected.

Powdery mildew: the powdery mildew fungus over-winters in the buds and when these start to grow in the spring it produces airborne spores which spread the disease. The pathogen originated in north America, but is now present wherever vines are grown. Leaf infection reduces plant vigour. When fruits are infected they split as they expand and this allows secondary infection by grey mould (Botrytis cinerea).

Grey mould: this pathogen produces abundant airborne spores on dead plant material, where it is able to grow as a saprophyte (feeding on dead organic material). It normally only infects plants through wounds, or particularly vulnerable organs (flowers or ripening fruit). When ripening grapes become infected they usually either fall prematurely or rot and then become mummified. When exactly the right climatic conditions occur, the so-called 'noble rot' develops, leading to fruit with a very concentrated sugar content which is used to make certain very desirable and expensive vintages; such conditions are, unfortunately, not likely to occur in the UK and are most unlikely in vines under glass.