Galls

Galls are the warts or tumors of the plant world.

Oak apples are a type of gall (Maksim) CC BY-SA 3.0

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Not really. Galls are neither warts nor tumors, but that’s how many of them appear. The word gall comes to us from the Latin galla, for ‘oak-apple’. Oak apples are not fruits. They are a plant’s reaction to the presence of a foreign substance. The study of plant galls is called cecidology [see-SID-ology]. Most commonly associated with baseball-sized knobs seen on oak trees, galls come in all sizes, and can be found on a variety of plants.

Lime nail galls (Rosser1954) Public Domain

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Gall description

Galls are swellings that occur in response to invasion. That invasion may be in the form of bacteria, fungi, insect larvae, eriophyid mites, nematodes, or other pests, and even other plants. Mistletoe is one example of a gall-forming plant. Unlike fungal cankers, which involve plant tissue death, galls, fungal or otherwise, are cases of extra tissue growth.


Galls are nearly always woody knobs that may occur on stems, branches, roots, buds, petioles, flowers, fruits, or leaves. Galls may be simple, with a single chamber (unilocular), or highly complex, with many chambers (plurilocular). Galls can also look like a sphere, a saucer, pineapples, pinecones, pouches, pods, or fantastic, tiny red spikes. It just depends on the host plant and the cause of the gall.

Root galls found on roses (Kate Russell)

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 Where they occur and what they look like inside can tell you a lot about what caused it.

• Bud galls, whose nectaries continue producing nectar, can be caused by insects, such as wasps. The nectar attracts protective ants and other stinging, biting insects.
• Fruit/flower galls are often caused by insects, such as wasps and eriophyid mites
• Leaf/petiole galls can appear on a leaf surface (aphids), the underside of leaf veins (wasp), or as dramatic red spikes on a leaf surface (midges).
• Stem galls can be caused by wasps. Stem galls, like bud galls, produce nectar. In rare cases, stem galls can girdle a tree.

Maple leaf galls (Jmeeter) CC BY-SA 3.0

If you cut a gall open, you will see distinctly arranged vascular tissues, depending on the cause of the gall, and an enlarged cambium layer. These distortions interfere with the flow of water and nutrients, which can lead to wilting and stunting. Or, you may see a large, open area, perfect for use as a larval nursery, with no noticeable impact to the host plant.


​Insect, mite, and nematode galls

When insects invade a plant, the galls that form are built by the insect. These galls can act as food or shelter for the insects. This is different from the plant-produced domatia (tiny apartments) found in some thorns for beneficial insects. Insect galls are made when an insect injects chemicals (pseudo plant hormones) into the plant, causing the gall to develop. Very often, eggs are laid in these galls, providing developing larvae with food and protection. Gall wasps, sawflies, gall flies, scale insects, some aphid species, weevils, psyllids, and gall midges can all cause insect galls, but it is nearly always gall wasps or gall midges.

Nematodes are microscopic round soil worms that can cause small galls on roots. Root knot nematodes are one such pest. In each of these cases, the gall is made up entirely of plant tissue, unlike fungal and bacterial galls, which incorporate fungal or bacterial tissues, respectively. Insect galls may also house interlopers, technically called inquilines.


Fungal galls

When a fungi infects a plant, it grows alongside plant cells, creating swollen areas that can develop into galls. Several varieties of rust can cause galls to form. When these galls form on conifers, as in the case of cedar apple rust, the galls look like gelatinous fingers, called telial horns.

Dried cedar-apple rust gall (Kate Russell)

Fungal galls on other types of leaves tend to look more spherical.

Eucalyptus leaf gall (fir0002) GFDL 1.2

Bacterial and viral galls

Bacterial and viral galls develop because the bacteria or virus reprograms plant cells into producing more bacteria or viruses, or other supportive cells. When galls are found at or just below the soil level, it is, most likely, crown gall. Crown gall is a bacterial disease that can occur on blackberries, sunflowers, grapes, and roses, along with almond, apple, apricot, cherry, and pear trees.


Galls on roots may mean clubroot, a disease caused by an entirely separate group of parasites, known as Phytomyxea. Root galls may also mean the presence of beneficial, nitrogen fixing Rhizobium bacteria.


Galls have long been used in tanning, to make ink, and as astringents. Most galls contains high levels of tannic acid and resin. There are even a few edible galls, the most notable being corn smut.


Sometimes, what looks like a gall is actually caused by herbicide overspray.


Galls are most commonly formed when plant tissue is new and undifferentiated. This meristem tissue is most often seen in spring, so that’s when you should start looking for galls. Once gall development begins, the tissues have been reprogrammed and cannot go back to normal.


Gall management

In a word, you can’t. Insect and mite galls rarely harm plants and you can’t completely control these pests anyway. Once they are inside the plant, there is nothing you can spray or apply that will even reach them. Anyway, the gall is already in place. Fungal and bacterial galls may, possibly, if you are really lucky, and can time it perfectly, be prevented or reduced with fungicide treatments. Or not.

Leaf galls on rhododendron (Xemenendura) CC BY-SA 3.0

If you are galled by galls, take them off. Otherwise, recognize that galls are just another amazing aspect of playing with plants.