The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
- Bud blast: remove and destroy infected flower buds promptly
- Powdery mildew or leaf spots: leaf removal is not recommended as it would cause severe defoliation. Unless very severe, these infections should be tolerated
- Galls should be removed promptly before the fungus sporulates (the white stage when the spores are released and the disease can spread)
Bud blast: control of the leafhopper, to prevent infection by bud blast, is difficult because they are strong fliers and reinvade rapidly, but sprays of natural pyrethrum (e.g. Bug Clear Gun for Fruit and Veg, Ecofective Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer), lambda cyhalothrin (e.g. Westland Resolva Pest Killer) or acetameprid (e.g. Bug Clear Ultra) from early August for as long as the pest is active may be helpful
Powdery mildew, rust and leaf spots: The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are approved for the control of powdery mildew and rust diseases on ornamental plants. They would probably also give some control of rhododendron leaf spots (Provanto Fungus Fighter Plus and Toprose Fungus Control and Protect mention 'leaf spot of ornamentals' on the label).
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.
Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
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