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;
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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.
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 leaf
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delboy on 05/07/2014
White spots on underside of leaves and stems of
my Rhododendron , plant in full leave growth no
sign of flowers.(White spots like cotton wool)
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