Archived Mountain Voices

Plant gall formation is somewhat of a mystery

mtnvoicesWhen it rains it pours. Within the past week or so, I received two emails about plant galls. That’s two more than I’ve received in the past 15 years of writing this column. Here goes.

Galls are timorous growths on leaves, stems, branches, trunks and roots caused by various agents. But they are usually induced by either insects or a fungus of some sort. 

One of the emails concerned an insect-induced gall sometimes called an “oak apple.” The other concerned a fungus-induced gall sometimes called an “azalea apple.”   

The exact manner in which insect galls are formed is something of a mystery. It used to be thought that the adult insect stimulated the gall at the time she laid her eggs on the host plant. Current accounts seem to lean toward the hypothesis that it’s actually the hatched insect larva that initiates gall formation. Once it commences feeding, the larva secretes an enzyme that breaks plant starch down into simple sugar, thereby stimulating plant cells to multiply in abnormal growth patterns.

More than 2,000 kinds of insect galls have been identified in North American plants, with 800 of these caused by gall wasps.  The shape that a gall assumes depends upon the species of insect that created it. Accordingly, galls are considered by many authorities to be a reliable criterion for distinguishing between closely related species.

In A Field Guide to Eastern Forests of North America (1988), biologist John C. Kricher theorized that, “Chemical irritation produced by boring larvae was probably the initial stimulation leading to gall evolution. Plants developed ‘tumors’ in response to the irritation caused by insects, and the insects adapted to use the galls to protect and feed developing larvae.”

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Insect galls are easy to find. Any stand of goldenrod will contain two basic types of stem galls: elliptical and round. Blueberry twigs display a reddish-brown, kidney-shaped gall that causes them to bend downward. A spiny gall can be found attached to witch-hazel stems and leaves.

Probably the most obvious and well-known type is the “oak apple” gall, which is a small tan ball 1 to 2 inches in diameter. These galls are found on the leaves of several oak species that grow on dry slopes throughout Western North Carolina.

I wouldn’t eat one, but some sources describe oak apples as being edible. From ancient times, they have also been a source of dyes and inks. Vincent van Gogh used oak apple gall ink (also known as iron-gall ink) to tint his drawings. 

The ink is still manufactured commercially by various labs that extract the tannic acids found in the oak apples by grinding and soaking them in water or wine for about a week, then adding ferrous sulphate to obtain a lustrous purplish-black ink. The high-quality ink obtained from the Aleppo oak gall of Asia Minor was at one time specified for use by the U.S. Treasury.

Round greenish spongy galls are often observed on the twigs of flame azalea. They are formed by the fungus Exobasidium vaccinii, a disease common to members of the plant family Ericaeae. These galls should be removed and disposed of by fire or in a lidded container so that the spores can’t be dispersed.  

In his Wild Shrubs and Vines (1989), Donald Stokes noted that: “The growths occur on the ends of the twigs and are about the size of a ping-pong ball. They look a little like the galls attached to oak leaves in spring, but instead of being stringy and tough, they are crisp, juicy, and sweet … They were more well known in colonial times and were even pickled and stored for later eating.”

I have eaten azalea galls, but I don’t recommend them. 

(George Ellison is a naturalist and writer. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..)

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