First of all, let's consider what a "gall" is, in general, in the context of horticulture. A gall is a tumor-like eruption in plant tissue. A number of different plants are susceptible. The two types of plants upon which you most commonly find galls are:
- Oak trees
A gall represents the plant's reaction to damage caused by an invading agent. In the case of azalea galls, that agent is a fungus (see below). But there are other agents that cause galls to develop on other plants. For example, one master gardener has written about the leaf galls caused by insects on various trees.
Regarding azalea leaf galls, specifically, you will notice two different phases (which may be present at the same time on the same plant, as in my picture):
- In phase one, the a whitish coating appears on swollen plant tissue. In spring and summer, the spores disperse and land on healthy leaves, both through air or by rain splashing on leaves. Cool, wet weather promotes the spread. The gall doesn't appear until the following spring.
- In phase two, it is a grayish-white blob (and more recognizable to the beginner as some sort of fungus)
All parts of azaleas are susceptible to galls: leaves, branch tips, flowers, and even seed pods can be infested with galls.
What to Do About Azalea Leaf Galls
First of all, take solace in the fact that their bark is worse than their bite. As alien-looking and repulsive as azalea leaf galls are, they are not considered overly serious. Wet, cool weather can cause the spread of the fungus, Exobasidium vaccinii. Pick off the azalea galls wherever they occur on the plant and dispose of them properly. Do not put them in the compost bin, and do not leave them lying around on the ground, lest the fungus spread to other branches.
For preventive care, remove galls immediately, but also the use of a fungicide may help prevent the development of galls. As always, good soil health and watering habits help keep plants healthy:
- Work ample amounts of humus into the soil to promote healthy, strong growth
- Apply landscape mulch around the plants to prevent wet soil from splashing on leaves
Furthermore, as is usually the case when dealing with fungus prevention, make it a point to apply water to your azalea shrubs down at ground level, rather than spraying from above. The latter practice gets the foliage all wet, which can promote fungi spread.
Having said all that, overall, I have found the amount of time I need to put into azalea care to be relatively minimal here in the Northeastern U.S. Care has consisted mainly of occasionally spraying neem oil on my Gibraltar azalea to kill aphids.