There are many species of powdery mildew, and these can affect outdoor and indoor plants.
Spores are spread by wind and rain splash, and the fungus may overwinter on host-plant surfaces.
The problem can cause plants to die and crops to become inedible.
The symptoms are seen as a white or light-coloured chalky residue on the leaves.
On rhododendrons it appears buff-coloured.
The powdery fungus may cover both the upper and lower surfaces of the leaves.
Infections cause distortion, chlorosis, and browning of the foliage, and sometimes premature leaf drop.
It generally affects lush new plant growth as opposed to mature growth.
Fruits such as grapes and gooseberries can be affected by powdery mildew.
In a greenhouse situation, frequent fluctuations in humidity as well as warm days and cool nights will favour the fungus as high humidity favours spore production, and lower humidity favours spore release.
Once the infection has started, it will continue to spread regardless of weather conditions.
Avoid growing susceptible plants on dry sites.
Ensure plants are regularly watered and don't want for moisture, especially those in containers.
Keep roots moist by incorporating organic matter in autumn and applying mulches in spring.
Humidity control in the form of good ventilation and spraying with clean water is a good defence against this disease.
Prune / Cut out affected areas as soon as possible, taking care not to shake the affected part/s, this will prevent further dispersal of spores.
Careful pruning and siting of plants outdoors will also help with the ventilation issue.
That is, dense growth or plants placed against a wall or fence can increase the risk of mildew occurring.
Powdery mildew can overwinter on infected material, so clear up plant debris around the garden prior to the onset of winter.
Fatty acid sprays can have some affect on contolling the disease.
The following treatment seems to work;
Spray affected plants with 1 part milk* (cows milk) to 9 parts water.
*Doesn't matter whether the milk is skimmed, semi skimmed or full fat.