Powdery mildew, just as the name suggests, is a white powdery fungus that grows mostly on the upper surface of leaves. It will occasionally spread to the underside and other parts of the plant. It prospers in dry soil conditions but where the overhead conditions are somewhat humid.
Plants growing in dry soil soil 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.
Leaves, particularly of young infected plants, may turn yellow and become distorted. 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.
Treatment and control
General tips: Remove and destroy all infected parts of the plant including any foliage on the ground.
Spray with a suitable fungicide: Products containing any one of the following active ingredients will be effective: myclobutanil, for example FungusClear 2 Gun! or triticonazole, for example FungusClear Ultra. Continue spraying at fortnightly intervals throughout the season.