Destroying fallen infected leaves in autumn will reduce the amount of infectious spores next spring. Mulching and watering reduces water stress and helps make plants less prone to infection. Promptly pruning out infected shoots will reduce subsequent infection.
Most powdery mildew fungi have a host range restricted to a relatively few, related plants, but these can include wild relatives which can be sources of infection, e.g. wild crab apples may be sources of infection for apple orchards.
Seed producers sometimes offer powdery mildew-resistant cultivars of both vegetables and ornamental plants, check catalogues for details.
Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.
Edibles: Tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus) can be used on glasshouse crops of cucumber, courgette and summer squash.
Ornamentals: The fungicides tebuconazole (Bayer Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus), and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are approved for the control of powdery mildews on ornamental plants.
The following products contain a combination of both insecticide and fungicide, enabling the control of both insect pests and disease: myclobutanil containing cypermethrin (Bayer MultiRose 2, Doff Rose Shield, Vitax Rosegarde, Westland Rose Rescue); tebuconazole containing deltamethrin (Bayer Multirose Concentrate 2), and triticonazole containing acetamiprid (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if pests are not a problem on the plants treated.
Any crop: The products SB Plant Invigorator, Resolva Natural Power Bug and Mildew Control, and Ecofective Plant Defender contain a blend of surfactants and nutrients and can be used on any edible or ornamental plants, with no harvest interval. They have a physical mode of action and may be used against powdery mildews, as well as a range of pests such as whiteflies, aphids, spider mites, mealybugs, scale insects and psyllids.
Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely