Galls

Galls are like warts or tumors in the plant world.

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

Not really. Galls are neither warts nor tumors, but that’s how many of them appear. The word gall comes 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.

Lime nail galls (Rosser1954) Public Domain

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 occur on many different plants.


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, 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 anywhere on a plant. Galls may be simple, with a single chamber (unilocular), or highly complex, with multiple pockets (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)

Where they occur and how they look inside can tell you a lot about what caused them.

• 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 and flower galls are in response to insects, such as wasps and eriophyid mites.
• Leaf and 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, which also produce nectar, can be caused by wasps. 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, leading to wilting and stunting. Or, you may see a large, open area, perfect for use as a larval nursery, with no noticeable impact on the host plant.


​Insect, mite, and nematode galls

When insects invade a plant, they build galls. These galls can act as food or shelter for insects. They are not the same as the plant-produced domatia (tiny apartments) found in some thorns for beneficial insects. Insects inject chemicals (pseudo plant hormones) into host plants, triggering gall formation. 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. These galls are 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 fungus 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, they look like glutinous 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, viruses, or other supportive cells. Galls at or just below the soil level are nearly always 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 parasites known as Phytomyxea. On the other hand, root galls may also indicate the presence of beneficial, nitrogen fixing Rhizobium bacteria. Galls have long been used in leather tanning, to make ink, and as astringents. Most galls contain high levels of tannic acid and resin. There are even a few edible galls, corn smut being the most notable. Sometimes, what looks like a gall is herbicide overspray.


New and undifferentiated plant cells are most vulnerable to gall formation. Spring is a good time to monitor plants for signs of galls. Once gall development begins, the tissues have been reprogrammed and cannot return to normal.


Gall management

In a word, you can’t. Insect and mite galls rarely harm plants, and you can’t control these pests completely, anyway. Once they are inside the plant, there is nothing you can spray or apply that will reach them. Anyway, the gall is already in place. 


Fungal and bacterial galls may be prevented or reduced with fungicide treatments if you can time it perfectly. Or not.

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

If you are galled by galls, remove them with a sharp knife. Otherwise, recognize that galls are just another amazing aspect of playing with plants.