Q What is azalea gall?

A Azalea gall is an abnormal growth produced by some azaleas and rhododendrons in response to attack by the fungus Exobasidium vaccinii. Very little is known about how the fungus enters the plant or why some plants are more susceptible than others.

Caption: Pick off any azalea galls you find

Q Which plants are likely to be affected by azalea gall?

A As its name suggests, only rhododendrons and azaleas are affected by this disease. Certain varieties seem more susceptible than others – if a variety seems to be getting hit particularly hard in your garden, then it may be worth replacing it with a different kind. Rhododendron simsii, the Indian hybrid or indoor azalea, is particularly prone to infection.

Q What does azalea gall look like?

A Individual leaves, and occasionally flower petals, are almost entirely replaced by a growth, varying in size from pea-sized to small plum-sized. This unsightly disease affects rhododendrons and azaleas both indoors and out. However, if your plants are growing well, they should resist attack The growth is fleshy and usually pale green, though sometimes it turns slightly reddish. As the fungus develops, a powdery white coating can be seen on the gall, which is a layer of white fungal spores. If the galls are not removed, they will finally wither and turn brown.

Q How does azalea gall spread from plant to plant?

A The spores of the fungus spread the disease from plant to plant. Spores may be carried on the air, transferred by insects or by physical contact.

Q What damage does azalea gall do to plants?

A Where only a few leaves are affected, there is little or no effect on the plant. If infection is severe, the plant's ability to manufacture food via its leaves will be reduced and the plant will be weakened. Good growing conditions, such as adequate moisture, good drainage, free air circulation around the branches and sufficient nutrients, will help to make plants more resistant to attack and better able to cope with infection.

Q Can I reduce the chances of my azaleas being affected by azalea gall?

A The azalea gall fungus is favoured by high levels of humidity, so good ventilation indoors or in the greenhouse, even in winter, will help to reduce the incidence of this disease. Keeping insect pests under control will also reduce the chance of the fungus being transferred between plants.

Q What can I do if my plants are affected by azalea gall?

A Galls that appear on azaleas and rhododendrons should be picked off and destroyed as soon as they are seen, before they turn powdery white and spores are produced. There are no chemical controls for this fungus.