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Notes From The Field Blog

All About Galls

Posted: 10/2/2013
Posted By: Bethany Majeski
Original Source: Notes from the Field

They’re all around you, though you may not have noticed: the papery brown balls kicked up by your feet on a forest trail, the odd-looking goldenrod that never did flower, but instead sports an unruly mop of spiky, pineapple-like leaves at its top, the almost-cute, fuzzy lumps on the surfaces of oak leaves that resemble benign little muppets. Often, the most amazing phenomena of nature surround us without our even being aware they exist. Such is the case with galls. Those oddly shaped lumps, bumps, folds, rolls, and grotesque protrusions found on plants everywhere this time of year are amazing microcosms enshrouded by mystery, history, and wonder.  There’s so much to say on this subject; for your reading pleasure allow me to break up the rest of this essay into parts which you can peruse according to your interest: the nerdy science background, common galls of our area, and the human history and value of galls.

microscopic mites are what make these fuzzy red patches on this maple leaf

The Nerdy Science Background

I find the best way to start learning a new subject is by uncovering its etymological history. The word ‘gall’ originates from the Latin galla, and means literally an excrescence found on trees. The French derivative of this became the verb galer- to annoy, fret, tease, or break the skin by chafing.  The study of galls is called ‘cecidology,’ from the Greek kēkis, even older than Latin and meaning something which is gushing forth, an oozing outgrowth, linked with the production of poison or bile. Eww...

Modern definition of the word ‘gall’ is an abnormal, localized outgrowth or swelling of plant tissue caused by infection from another organism. Galls vary greatly in size and shape, from single enlarged cells to complex multicellular growths with vascular tissues.  Galls occur all over the world, and 99% of them occur on angiosperms- flowering plants (for the botanists out there, 90% of galls occur specifically on dicotyledons). In North America, 50% of galls occur on beech and oak trees, 20% occur on members of the Asteraceae family (think sunflowers, goldenrods, daisies, etc.) and 15% occur on berry-bearing brambles, cherries, apples, and other members of the rose family.

So who makes all this strange growth occur? Galls are made by a hodge-podge of different organisms. The simplest galls, woody and dense in structure without any recognizable chambers or organized form, are caused by viruses and bacteria. Fungal galls, too, are often simple and solid in form; however, some put on a real show when producing their reproductive spores. Arthropod galls- mostly caused by midges, flies (Diptera) and wasps (Hymenoptera), but also aphids, adelgids, phylloxerans, psylloids, scales (Hemiptera), beetles (Coleoptera), and moths (Lepidoptera)- are among the most complex and fascinating galls out there. In any case, to cause a gall, the gall-maker must affect meristematic cells- these are omnipotent, like our stem cells- which are found in a plant’s buds and cambium tissue of the stem, leaf veins, and roots. In order to truly be classified as a gall, the gall-maker must cause change in these cells, chemically catalyzing them to grow in a way they ordinarily would not; the gall-maker causes the abnormal growth then benefits from it in the way of food or shelter. This is what distinguishes a true gall from, say, the bumpy scar tissue formed on a tree’s trunk in a response to being wounded by a buck’s antlers.

Cedar apple rust before rain

Cedar apple rust after rain- wow!

For most of the year, this Gymnosporangium gall-shown on redcedar, a common host- looks like a nondescript brown ball. When fruiting, however, it produces spore-bearing, alien-like orange jelly tentacles called "telial horns"

Almost always, the relationship between plant and gall-maker is parasitic (interesting exceptions to this rule can be found below in The Human History and Value of Galls). Very often, however, the gall-maker causes only aesthetic damage, with the small amount of energy stolen having little effect on the plant’s overall health. This is not to say galls can’t cause damage; some bacterial, fungal, and viral infections can kill otherwise healthy, mature trees. The worst agricultural plagues often stem from gall infections- brassica clubroot, crown gall disease, grape phylloxera, and more.

bacterial, fungal, and viral galls are common causes of tree burls

You know what the most fascinating aspect of gall science (cecidology!) is? No one knows specifically how galls are made. The processes by which bacterial galls form is believed to be understood by science, but as for those multi-chambered, fancifully-colored blobs and bumps that look like they were dreamt up by the Warhol Factory? The jury is out. What do scientists know? That the gall-maker redirects normal growth of the plant via chemical and mechanical processes, and that these processes probably vary across the different groups of gall-maker. Most books on galls don’t even touch on the subject of how they are made. Don’t you love a good nature mystery?

Common Galls of Our Area

Want to wow your friends with your nature knowledge? Learn just one or two common galls and you’ll have everyone wondering  how you got to be so smart. Look for galls in mid-summer to late fall. While they are present at other times of year, most arthropod galls reach their height of maturity during the harvest season.

Wooly Oak Gall

Everyone wants to know what these fuzzy little blobs are! Found on the leaves of oak trees, these eye-catching galls are the nursery chamber for a small wasp. As you will see, many of these wasps are responsible for our more familiar galls.


Oak Apple Gall

The quintessential gall of our area. There are different types of oak apple galls, but the one we most commonly see is hollow and papery. A female wasp lays an egg in the tissue of an oak leaf, and the feeding of the developing larva stimulates the leaf to form the shape of a sphere- a perfect wasp nursery!


Huitlacoche (whee-luh-COE-chay)

This fungal gall causes corn kernels to swell ghoulishly and turn a grayish-white. Considered a scourge to U.S. farmers and referred to as “corn smut,” huitlacoche has long been a favorite delicacy in Latin America.

This looks gross, but is supposed to taste great!

Witch’s Hat Gall

This unmistakable gall is caused by an aphid. Its shape an visibility during the fall contribute to its spooky name.

conical and cute- just like little witches' hats!

Cedar Hawthorn Rust

I just saw this fungal gall on some hawthorns in Huntington Reservation. Blotchy patches are visible on the top surface of the leaf, but flip it over and you’ll see the wavy little fingers of the spore-bearing structures!

Goldenrod Ball Gall

You’ll find this gall in any field with goldenrod.  A fly deposits her egg inside the stem of a goldenrod, and the developing larva causes a spherical gall to form. The larva overwinters and emerges in spring, but many times you’ll see evidence of hungry birds pecking holes into the galls to steal the fat-and-protein rich grub from its chamber. 

This goldenrod ball gall has been infiltrated by a hungry bird looking for the nutritious larva within

You can easily find goldenrod ball galls in fields this time of year

Cherry Pouch Gall

Finger-like projections house a developing mite. You can see this common gall on wild black cherry trees in mid-summer.

The Human History and Value of Galls

While they may just seem like bizarre anomalies of nature, galls share quite an extensive history with us humans. What do the original manuscripts of Bach’s compositions, DaVinci’s doodles, the U.S. Constitution, and the Dead Sea Scrolls all have in common? They were written with iron gall ink! That’s right, a type of oak apple gall (caused by a wasp) was historically pulverized, mixed with “vitriol” (an iron/sulphur compound), and wine. The resulting slurry was used to pen some of the most important written works known to our culture.

oak apple galls and ferric sulphate are the ingredients behind some of history's most important documents

That wasn’t all galls were used for. Greek physician Hippocrates and Roman philosopher Pliny the Elder both touted the benefits of various galls to cure ills ranging from infected wounds to baldness! In fact, medical scriptures from all over the world have made reference to galls as cures and tonics. Many scientists believe that the naturally high tannin contents of many galls may very well have been useful in treating lesions and infections.  This high tannin content also made galls useful in the preservation of animal hides.

Perhaps the most intriguing story from history tied to galls is that of St. Anthony’s Fire or Ignis Sacer.  In the Middle Ages, people often fell sick with a mysterious illness. It caused terrible, burning pain in the limbs and extremities, followed by icy cold. Those afflicted suffered terrible convulsions of the arms, legs, feet and hands, and the circulation was so restricted that the body parts often turned gangrenous and fell off. People suffering from this “Holy Fire” were known to go into maniacal fits, and see and hear things that weren’t there. Many traveled to see the monks of the order of St. Anthony, who reputedly were able to treat and cure the disease.

Not until the mid 19th century was the true cause of St. Anthony’s Fire discovered- poisoning from a fungal gall called ergot. Growing as swollen black lumps on rye, barley, and other grains, ergotism poisoned millions of people across centuries of early agriculture. In recent times, derivatives of this gall have been used to create the hallucinogen lysergic acid diethylamide, or LSD. It has been suggested that ergot poisoning may indeed have been responsible for the hysteria and “bewitchment” affecting early Massachusetts during the Salem Witch Trials.

An ergot gall growing on barley

So do galls do us any good? Absolutely. There are three known examples of galls acting as beneficiaries to their hosts- and us. Take the fig. The relationship between this fruit and its pollinator has got to be about the most complicated thing you can imagine. In fact, you really need a picture to be able to understand how it all happens. Suffice it to say that a tiny wasp climbs inside the unripe fruit, which houses the male and female flowers of the fig, and lays her eggs on these. The flowers form a gall in which the larvae develop, and when they hatch out, go through a complicated, perfectly-timed pollination/reproduction sequence. Trust me, look it up and you’ll never think of figs the same again.

Galls also play a huge role in the overall health and functioning of our ecosystems. Take the nitrogen-fixing bacteria that form gall nodules on the roots of plants in the legume family. The bacteria capture atmospheric nitrogen and turn it into a form the plant can use as fertilizer, and in return the plant provides moisture and nutrition to the bacteria. These important galls can organically enrich soils when legumes are planted as cover crops on agricultural fields.

these tiny bacterial galls allow plants in the legume family to fix nitrogen from the air

In the world’s forests, fungus silently rules. Fungus does good work in the form of decomposition, but its regenerative and restorative powers are what sustain our ecosystems . Of all the plant families thus far studied for these relationships, 95% have been shown to  have mycorrhizal galls, which form between fungi and plants’ roots. The fungi benefits from having a steady supply of carbohydrates like glucose and sucrose, while the plants in turn benefit from the greater absorptive abilities of the fungi. Plants with mycorrhizal galls are shown to be more resistant to drought and disease, and are protected from soil contaminants and pH fluctuations. So greatly important is this tie between fungi and plants that when people try to replant on barren land abused by industry or agriculture, they often must inoculate the soil with mycorrhizae so that the plants can grow.

None of our beautiful, mature forests would exist without mycorrhizal fungus!

Ubiquitous yet unique, fascinating but freakish, galls are all around us. I hope that your discovery of them will light the fire for future observations and adventures within Cleveland Metroparks!

Bethany Majeski


10/29/2013 8:03:51 PM by
10/10/2013 2:22:46 PM by Bethany Majeski
Hi Judy-
Glad you enjoyed reading the article. It was hard to pick which galls to write about- there are so many to choose from! The acorn plum gall is very closely related to oak apple galls, with both being formed by a cynipid wasp (Amphibolips quercusjuglans is the species for the acorn plum, Amphibolips confluenta is the oak apple). Oak apple galls form on or at the base of an oak leaf and are hollow, whereas acorn plum galls form on an acorn or twig with acorns on it, and, as you mentioned, are fleshy and juicy!
So glad to hear you are discovering cool things in the park!
10/9/2013 9:00:15 PM by Judy Schuster
I guess I am a nerd in that I enjoyed reading this. I found an Oak Plum Gall in Bedford Metropark a few years back. I took it home and cut it in half... was like a juicy plum inside. I was intrigued by this odd find and later found out what it was. Surprised you didn't mention this particular gall in your story. I would be interested to read what you have to say about addendum to the current story perhaps ?
10/9/2013 2:52:16 AM by Tom Lacerda
amazing! I've studied goldenrod gall formation in college. I was so surprised to find all the bizarre different interactions and defenses this seemingly benign feature of our landscape had to offer. Especially neat was finding adult parasitoid wasps inside the galls, dead, with no exit tunnel. They had pupated in their host, killing them, before they had dug the necessary exit tunnel!

Thanks for writing this, I learned a lot!
10/8/2013 11:43:08 PM by Samuel Getts
Interesting article
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