Rhododendron diseases

What are rhododendron diseases?

The most common specific fungal diseases that affect rhododendrons include;

• Bud blast (Pycnostysanus azaleae)
• Powdery mildew (Erysiphe spp.)
• Petal blight (Ovulinia azaleae)
• Azalea leaf gall (Exobasidium japonicum)
• Leaf spots (Glomerella cingulata)
• Rust (Chrysomyxa ledi var. rhododendri)

They are also attacked by more generalist diseases such as;

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

Symptoms

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, infections on the lower leaf surface can be hard to detect, but tend to cause pale green or yellowish patches on the upper surface
• On deciduous azaleas, white powdery fungus grows over the upper leaf surface

Petal blight:

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

Galls:

• 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

Rust:

• Yellow 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 occurs

Silver leaf:

• Causes dieback of the branches

Control

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)

Chemical control:

• 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 (Bayer Sprayday Greenfly Killer) or lambda cyhalothrin (Westland Plant Rescue Fruit and Vegetable Bug Killer) from early August as long for as the pest is active may be helpful
• Powdery mildew or leaf spots: fungicides containing difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter and other formulations), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra, Roseclear Ultra) are are approved for control of powdery mildews and rusts on ornamental plants and would probably also give control of leaf spots.The formulation of tebuconazole (Bayer Garden Multirose Concentrate 2) contains deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid to control insect pests. Avoid these unless an insect pest problem is specifically identified.
• Petal blight: there are no chemical treatments for petal blight

Downloads

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

Biology

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.