Growing vines either outside or under glass can be most rewarding, but there are various problems that can commonly affect plants and fruit during the growing season
Images: Tim Sandall
Powdery mildew
This is the most important disease of grapes under glass in the UK. The fungus Erysiphe (syn. Uncinula) necator causes white powdery patches on
leaves and fruits and reduces vigour and
yield. It causes fruit to ripen unevenly and split, becoming infected by grey mould.
The fungus persists over winter in fallen leaves, or on the vines, spreading in the growing season via airborne spores. High humidity favours disease development, as does soil dryness; keep vines moist and well ventilated.
The only fungicide available to gardeners is sulphur dust, which can be difficult to apply effectively overhead. Recently available products containing plant and fish oils will also probably give some control.
Grey mould
The fungus Botrytis cinerea which causes grey mould is an ubiquitous saprophyte, growing on dead plant material, but can also invade plant tissues through wounds. If grape fruits have been damaged by powdery mildew they are more likely to be colonised by grey mould, causing them to shrivel. Fluffy grey fungal growth will be apparent if humidity is high. Infections are initiated from airborne spores.
It is important to clear dead leaves and other material to reduce spore numbers. There are no fungicides available to home gardeners for grey-mould control, but the measures suggested for powdery mildew control will minimise damage.
Downy mildew
Caused by fungus Plasmopara viticola, this foliar disease of outdoor vines is unrelated to powdery mildew. White patches of fungus appear on undersides of the leaves, limited by the veins.
It is spread by wind, but needs water on the leaves to infect. It is seldom serious in the UK, which is fortunate as no control is available.
Shanking
This is a disorder in which fruit grown under glass fails to ripen or develop colour. It then shrivels and takes on an unpleasant flavour.
This problem is caused by a poor root system, due usually to waterlogging, soil compaction and poor soil nutrition. Foliar feeding and careful soil improvement are the only controls.