There are many different fungi that cause this problem, each of which usually affects a particular plant or group of plants. In each case they produce similar symptoms. Affected plants will have discoloured leaves, often also distorted when young, while general growth and vigour is poor. Also learn about powdery mildew on roses.
In summer, a white powdery coating covers the leaves and possibly the shoot tips and flowers too. Leaves may then become discoloured and distorted, with poor growth and even dieback.
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Prune out affected parts of the plant and destroy. Healthy plants are less prone to attack, so make sure you water regularly and mulch around them to conserve moisture in the soil. Space out your plants to avoid over-crowding and thin out congested growth to improve the airflow. Also minimise the use of high-nitrogen fertilisers, as these lead to lots of soft, leafy growth, which is vulnerable to an attack. Get rid of weeds as they can spread the fungus.
Spray ornamental plants with sulphur. Always check the label first if you're using these products on edible crops.