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;

• Honey fungus (Armillaria spp.)
• Phytophthora root rot (Phytophthora spp.)
• Silver leaf (Chondrostereum purpureum)

Bud blast:

• Flower buds go brown and die but remain attached
• Later, the buds may turn silvery grey before becoming covered in small black bristles

Powdery mildew:

• 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

Petal blight:

• Spots appear on petals under wet conditions, which spread rapidly and cause the petals to collapse in a wet, slimy mess

Gall:

• Usually on azaleas, the leaves become very pale, swollen and distorted, then covered in a bloom of white fungus spores

Leaf spots:

• 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

Rust:

• 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

Silver leaf:

• 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.

Non-chemical control

• 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)

Fungicides

The RHS recommends that you don't use fungicides. Fungicides (including organic types) may reduce biodiversity, impact soil health and have wider adverse environmental effects. If you do intend to use a fungicide, please read the information given in the link and download below to ensure that use, storage and disposal of the product is done in a responsible and legally compliant manner.
  
The products listed in the ‘Fungicides for gardeners’ document below are legally available for use by home gardeners in the UK. This information is provided to avoid misuse of legal products and the use of unauthorised and untested products, which potentially has more serious consequences for the environment and wildlife than when products are used legally. Homemade products are not recommended as they are unregulated and usually untested.

There is no specific information available on the efficacy of any home garden fungicide against fungal leaf spots of rhododendron. No products are available to home gardeners with activity against petal blight, gall, root diseases or silver leaf.  Control of the leafhopper to help prevent infection by bud blast is difficult because they are strong fliers and reinvade rapidly, but information on control measures can be found in this profile on the insect.

Downloads

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Link

Chemicals: 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 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.