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.
Scientific name Various
Plants affected Rhododendron and deciduous azaleas
Main causes Fungi
Timing All year
What are rhododendron diseases?
The most common specific fungal diseases that affect rhododendrons include;
- Bud blast (Seifertia azaleae)
- Powdery mildew (Erysiphe spp.)
- Petal blight (Ovulinia azaleae)
- Azalea leaf gall (Exobasidium japonicum)
- Leaf spots (predominantly Colletotrichum gloeosporioides, but occasionally other fungi)
- Rust (Chrysomyxa rhododendri)
They are also attacked by more generalist diseases such as;
- Flower buds go brown and die but remain attached
- Later, the buds may turn silvery grey before becoming covered in small black bristles
- On evergreen rhododendrons, actual growth of the fungus on the lower leaf surface can be hard to detect, but infection tends to cause pale green, reddish-purple or yellowish patches on the upper surface
- On deciduous azaleas, white powdery fungus grows over the leaf surfaces
- Spots appear on petals under wet conditions, which spread rapidly and cause the petals to collapse in a wet, slimy mess
- Usually on azaleas, the leaves become very pale, swollen and distorted, then covered in a bloom of white fungus spores
- Purple or brown, more or less round spots appear on the upper leaf surfaces, usually of evergreen species. They sometimes contain numerous small, black fruiting bodies of the causal fungus
- Discoloured spots on the upper leaf surface correspond to pustules containing dusty orange spores on the lower surface
Honey fungus and Phytophthora root rot disease:
- Sudden wilting and collapse of the whole plant can occur, or there can be a slower and more gradual decline
- Causes dieback of the branches, with fruiting bodies of the fungus sometimes developing on the dead or dying parts
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 infection on control measures can be found in this profile on the insect.
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).
Sulphur (Fungus Clear Ultra 2, Mildew Clear for Edibles) can be used against powdery mildew.
SB Plant Invigorator, Growing Success Fungus Stop, RHS Bug and Mildew Control, Ecofective Bug & Mildew Control and the Ecofective ‘Defender’ range 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. They are unlikely to be effective against leaf spots.
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.
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 potentially 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 this group of diseases.
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 fungus produces airborne spores on the white bloom that develops on the 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 water-dispersed spores from fruiting bodies on the infections.
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.
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