Downy mildew is a fungal disease that is unrelated to powdery mildew and only affects outdoor vines.
It is spread by wind, but needs water on the leaves e.g rainwater / dew to infect the plants.
The symptoms are white patches of fungus that appear on undersides of the leaves,and correspond to light yellowish patches on the surface of the leaves.
The problem is seldom serious in the UK, which is fortunate as no control is available.
The spores unlike powdery mildew spores grow on dead plant material.
Meaning good hygiene around the plants is often the best control i.e. keeps the area around the plant/s free from dead leaves and detritus, particularly if the plants have been affected by powdery mildew.
The symptoms initiated by airborne spores, are seen as fluffy fungal growth on the fruit, the fruit becomes soft and discoloured.
The fungal growth tends to appear when humidity is very high.
Control: Keep vines under glass well ventilated and watered.
Well pruned trees can improve air circulation around plants.
Avoid overhead watering as this can lead to increased humidity.
Mulching to retain moisture can ensure that the plants are never stressed due to moisture starvation.
There are no fungicides available to the amateur gardener to control this disease, but by using the treatment suggested for powdery mildew, damage can be minimized.
Reducing the humidity around the plants can also help to keep the number of spores down.
Grapes grown under glass are the most susceptible to Powdery mildew.
During the winter months; spores dwell in fallen leaves or on the vines, then in spring these are spread by the wind / draughts to infect the new emerging leaves.
Symptoms: The fungus is seen as white powdery patches on the leaves and like rust, only lives on living tissue.
The general health of the plant/s deteriorates affecting the yield and quality of the fruit to the point that it can split and become infected by grey mould.
Control: To control the problem always keep the vines moist and well ventilated at all times during the growing season.
Drought conditions can stress the plants and make them more susceptible the disease.
The only fungicide available to gardeners is sulphur dust, which can be difficult to apply effectively overhead.
This is a disorder generally affects grapes grown under glass.
Symptoms: The symptoms are shrivelled fruit that fails to ripen and has an unpleasant flavour.
This problem is caused by poor root systems.
Plants should never be allowed to become pot bound.
They should never be overwatered as this can cause water-logging, over compaction and / or, it may wash out the soil nutrients.
Foliar feeding and careful soil improvement is the only control.
These small mites are serious pests of plants grown in greenhouses and in the home,and is a common pest with grapes.
Control: The best cure is prevention, and this can be done by creating adverse conditions for the pest.
The mites thrive in hot, dry conditions so misting the plant will increase humidity, however humidity can also hamper the quality of the berries.
The adult and immature mites feed and breed on the undersides of leaves, causing a fine light mottling of the upper surfaces.
In severe infestations, this leads to extensive yellowing and bronzing, followed by leaf fall.
The withering leaves are often covered with fine silk webbing.
The mites cease breeding in the autumn, and female mites over-winter in crevices in brickwork and woodwork, in bamboo canes, stakes and plant ties.
Reduce the numbers of over-wintering female mites by clearing out plant debris and by eliminating over-wintering sites in late autumn and early winter.
In the greenhouse, regular fumigation will prevent serious increases in mite populations.
Thorough spraying with a suitable pesticide, will protect plants under glass and outdoors.
The mites are likely to develop resistance to a given chemical if it is used too regularly, and control measures should be varied as much as possible.
Pesticides approved for use by amateur gardeners against red spider are limited to organic pesticides such as rotenone (derris), fatty acids (soft soaps) and vegetable oils.
The last two work by blocking the mites breathing tubes, preventing respiration.
The use of the biological control, i.e. the use of predatory mites, is proving to be quite a successful and is now widely used in preference to pesticides.
Grapevines can be susceptible to this root disease.(see here)