Azalea gall

Azalea gall attacks certain rhododendrons and azaleas out of doors and is often seen on indoor pot-grown plants of Rhododendron simsii, the popular Indian azalea, obtained from florists.

Azalea gall

Quick facts

Common name Azalea gall
Scientific name Exobasidium japonicum
Plants affected Rhododendrons, mainly azaleas
Main symptoms Galls (fleshy swellings) on leaves or flowers
Caused by Fungus
Timing Spring until autumn

What is azalea gall?

This is a common disease caused by the fungus Exobasidium japonicum that disfigures, but does not kill, rhododendrons including azaleas. It spreads by airborne spores.

Symptoms

Azalea gall is a very distinctive and curious-looking disease;

  • Galls varying in size from that of a pea to a small plum form on the leaves or flowers
  • The leaf or flower involved is practically replaced by the fleshy irregular gall
  • Galls are at first pale green, or very rarely reddish
  • Later they become white owing to the formation of a floury bloom, which is a superficial coating of fungal spores

Control

Non-chemical control

  • Remove and dispose of the galls, ideally before they become white and therefore infectious
  • Indoors, avoid an excessively moist atmosphere after the disease has appeared
  • Some cultivars are especially susceptible and replacement is the best option where a plant is heavily and persistently affected

Chemical control

There are no fungicides available to amateur gardeners for the control of azalea gall. It is thought that some insect pests may spread the spores of the fungus, and these should be controlled with an insecticide if possible.

Biology

The spores of the azalea gall fungus are insect or air-borne, and spread the disease from infected to healthy plants. On germination of the spores the fungus enters the tissues of the host, but can then have a long incubation period so that symptoms may not appear until several months after infection. However, very little detail is known about the life-cycle of the fungus.

A closely-related fungus causes a similar disease on camellias.

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