A full discourse on vine diseases is beyond the scope of this site; the most common diseases and conditions under which they are likely to develop are:

Disease Condition Comments
Phylloxera Learn about phylloxera (daktulosphaira vitifoliae)
Botrytis Warm, humid. Learn about botrytis (botrytis cinerea)
Warm, low light, humid Learn about powdery mildew (oidium)
Dead arm Rain when pruning. Learn about dead arm (eutypa dieback)
Warm and wet. Learn about downy mildew (plasmopara viticola)

The difference between powdery and downy mildew is not obvious, especially to new growers. Powdery mildew is much more common in the UK and is the likely culprit.
The Department of Agriculture, Western Australia (see link below) describes the ‘bag test’ to identify downy mildew (plasmopara viticola), as follows:

The bag test is used to indicate active downy mildew.

Pour clean water into an empty sealable plastic bag; shake the contents, then empty to leave the bag lightly moistened. Do not leave the bag wet or dry.

  1. Collect fresh, suspect leaves or bunches and place them in the sealed bag overnight in the dark at 20°C to 25°C, but at least greater than 13°C. A kitchen cupboard is usually ideal. Do not place leaves or bunches on top of each other. It is best to place the leaves facing downwards in the bag.
  2. Next morning, white down will have developed on young bunches or on the undersides of oil spots if downy mildew is present.

Spraying vines

Most professional growers spray vines to prevent and, if necessary, cure diseases; amateurs do not have so many options. » More.

Diseases - Some useful links:

1. Grapevine Diseases in New Zealand
2. South Australian Research and Development Institute
3. Department of Agriculture, Western Australia
4. Penn State University
5. Missouri State University

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