Read the whole leaf and the bunch
Downy mildew produces yellowish, sometimes translucent patches on the upper leaf surface, with whitish growth beneath. Powdery mildew produces a whitish-grey surface coating on foliage and berries; infected young grapes can split as they expand. Grey mould develops a fuzzy grey-brown growth on damaged or rotting tissues. Check the underside of leaves and the berries within the bunch.
Separate primary damage from the organism occupying it later. A berry split by powdery mildew can subsequently be infected by Botrytis.
Use the crop's setting in the diagnosis. Wet conditions favour grape downy mildew, which is seldom seen on vines under glass.
Reduce overwintering downy mildew and open the canopy
Remove infected fallen foliage, because the pathogen survives winter in resting spores in dead leaves. During the growing season, keep the canopy open through pruning and training, so that no compact pockets of foliage remain around developing bunches.
Powdery mildew can develop without a wet leaf surface
Powdery mildew fungi produce airborne spores with unusually high water content. They infect under drier conditions than many other fungal pathogens. Humid, stagnant air favours infection, even though free water is unnecessary. A dry leaf does not rule out powdery mildew.
Improve airflow by keeping growth sufficiently open. Remove infected shoots promptly and clear infected fallen leaves at the end of the season. Avoid excessive soft growth encouraged by overfeeding. Powdery mildews have narrow host ranges: the mildew on a neighbouring pea crop is a different organism from apple mildew.
Grey mould control starts with wounded and decaying tissue
Botrytis cinerea commonly attacks wounded or stressed tissue and spreads rapidly under humid conditions. Remove affected material promptly, keep infected debris away from healthy plants and reduce the humid conditions that favour spread.
Keep your spray records in one place. Free to start.
Under protection, ventilation and avoiding overcrowding help lower humidity. Water so that foliage can dry, and avoid damage during canopy work and picking.
Best insecticide for grapes: first identify the fruit-fly problem
Spotted wing drosophila, Drosophila suzukii, is a separate grape protection issue. Table grapes are among the susceptible crops; damaged wine grapes are also at risk. Wild blackberry is an important UK host, so monitoring includes the crop edge and nearby host vegetation.
Begin seasonal monitoring early, including woodland and hedgerows, and follow population movement towards the crop as fruit develops.
Remove overripe, damaged and waste fruit throughout harvest. These materials support the pest and can remain after the marketable bunches have been picked. Continue hygiene to the end of picking.
Match the chemical decision to the exact grape use
Search the HSE register using the product identity and the intended crop use. The professional and amateur search routes are distinct; a home vine uses the garden-product route.
The product documents give the crop or situation, maximum application conditions, latest application timing and protective measures.
Tracing affected bunches
The batch traceability guide covers tracing a lot back to its row and date.
Traps for spotted wing drosophila
For small-scale monitoring, bottle traps baited with apple cider vinegar are one option; purpose-built traps and sticky cards are also available. Check for adults weekly and renew liquid bait or cards on the stated schedule. Place traps near the host, including crop edges, hedges and wild fruit.
Next step
Read about fruit orchard disease control, look up a product in the HSE register guide and trace lots with batch traceability.