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Rhododendrons (including deciduous azaleas) may be attacked by several diseases, causing unsightly foliage or a lack of flowers. The most significant are powdery mildew and bud blast.
The most common specific fungal diseases that affect rhododendrons include;
They are also attacked by more generalist diseases such as;
Bud blast: control of the leafhopper, to prevent infection by bud blast, is difficult because they are strong fliers and reinvade rapidly, but fortnightly sprays of deltamethrin (e.g. Bayer Sprayday Greenfly Killer), lambda cyhalothrin (e.g. Westland Resolva Bug 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 (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 mildew and rust diseases on ornamental plants. They would probably also give some control of rhododendron leaf spots (Bayer Fungus Fighter Plus mentions '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 sprayerChemicals: using safely and effectivelyChemicals: storing and disposing safely
Bud blast: the fungus causing bud blast produces its spores on the black fungal 'bristles' that appear on infected buds. They are spread by water and infect buds through wounds made by the rhododendron leafhopper when it punctures the bud to lay its eggs (although some recent research has cast a doubt on this link between the pest and disease).
Powdery mildew: the biology of powdery mildew is discussed in the web profile devoted to them.
Petal blight: the fungus causing petal blight produces wind-dispersed spores on old infected flowers which remain hanging on the plant from the previous season.
Gall: the azalea gall fungus produces airborne spores on the white bloom which develops on galls. These may lodge and develop in buds in a similar way to the peach leaf curl and pocket plum pathogens, although they belong to a different group of fungi. Little is known about this pathogen.
Leaf spot: the leaf spot fungus produces spores from fungal structures on the infections, which are dispersed in water.
Rust: the rust pathogen releases airborne orange spores which spread the disease among Rhododendrons, then later it produces dark resting spores. When these germinate they infect the alternate host which is spruce (Picea spp.). Spores produced on spruce then reinfect rhododendrons.
Azalea gallBrown leaves on woody plantsHoney fungusPhytophthora ramorum and P. kernoviaePhytophthora root rotPowdery mildewRhododendronRhododendron leafhopper and bud blastSilver leafWhy has my tree or shrub died?
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