Mildews
Mildews are a fungal disease that affects a wide range of plants, especially those that are stressed or have become dry at the roots. The symptoms are light grey powdery patches on the leaves, shoots and flowers, sometimes turning brown as they mature. The lower leaves are often the most affected, but mildew can appear on any part of the plant that shows above the ground. As the disease progresses, the spots get larger and thicker as massive numbers of spores form, and the mildew spreads up and down the length of the plant.
Powdery mildew remains on the surface of the plant, whereas downy mildew often penetrates inside the plant, eventually killing it.
Control
Mildew can be prevented by keeping plants regularly watered and mulched so that they remain healthy and are able resist infection. Avoid overcrowding plants and ensure that they receive adequate ventilation; poor circulation of air is one of the reasons why climbers on warm walls are particularly prone to attack.
Resistance to powdery mildew has been bred into some roses and gooseberries, and may also be prevented by spraying with propiconazole or carbendazim. Downy mildew may be prevented by spraying with mancozeb or organic sprays, such as those made of nettles or garlic.
All infected growth should be removed and burned - do not put it on the compost heap.