Powdery mildew disease is common on a wide range of plants. There are numerous forms of powdery mildew, which are specific to one plant or a group of plants, and won't spread to other plants. For instance, rose powdery mildew attacks roses, clematis powdery mildew attacks clematis - they won't spread to each other.
Powdery mildew, as the name suggests, is a powdery fungus that grows mostly on the upper surface of leaves. It is usually white in colour. It will occasionally spread to the underside of leaves and other parts of the plant, such as stems and sometimes flowers and fruit.
Powdery mildew prospers in dry soil conditions, but where the overhead conditions are somewhat humid or damp.
Leaves, particularly of young infected plants, may turn yellow and become distorted and finally turn brown. Infected areas and indeed whole leaves will gradually die back and dry off.
On rhododendrons, the fungal growth is still powdery but may be beige in colour and may occur more frequently on the underside of the leaf.
Remove and destroy all infected parts of the plant including any foliage on the ground.
Plants growing in dry soil/compost conditions that are under stress will be more prone to infection. So, keep susceptible plants well watered and mulch the soil with bark or similar mulching material. Only water the soil, not the foliage as the disease spores need a layer of moisture on the leaves to grow.
Spray ornamental plants thoroughly with a systemic fungicide. Continue spraying throughout the season as recommended on the label.