Plant galls, growths of various origins

In the shape of an apple, an artichoke, an ear, a berry, a wine bottle, a purse or even a horn, vegetable galls take on various aspects that are always very worrying for the gardener. The stems, buds and leaves are then covered with these growths which raise very legitimate questions.

Eriophyes tiliae, a mite responsible for the formation of the horned gall of lime
Eriophyes tiliae, a mite responsible for the formation of the horned gall of lime

What is a gall?

Otherwise named ‘Cécidie‘ the galle is a cellular response of the plant to external attack. The protuberances visible on the plants are the result of a modification in the process of development of the tissue resulting from this attack.

Galls can be of multifactorial origins.

Most often, it is a simple reaction to the insect bite or a biting / sucking mite, in other rarer cases, they reveal the presence of a fungus, bacteria or virus.

The deformations observed can take on many shapes ; feathery in the hairy gall or ‘bédégar’, in a helmet, in a ball, in a berry, forming small lenses or strange warts, it can also take the form of a small apple, an artichoke, a walnut and even a shrub!

This is more than enough to leave the gardener perplexed when he sees an ‘apple’ on a leaf of oak, red horns on the leaves of its lime tree or a colorful and tousled pompom on its rosebush!

Note that galls can form as well on the stems, leaves, fruits and buds as on the roots, they are then less visible. The latter are usually caused by nematodes of the genre Meloidogyne, otherwise named ‘Root-knot nematodes‘.

How is a gall formed?

As we have seen, the gall appears during an external attack, often through a sting. In this case, insects or mites take the opportunity to lay their eggs in the plant tissue. The chemical process initiated by the plant to defend itself produces gall which will serve most of the time as a shelter for the larvae developing inside. The said larvae will then feed very quietly on the sap and plant tissues without being able to be reached by predators!

Certain fungi can also be responsible for the appearance of galls which are then named ‘Mycocécidies‘. They penetrate the tissues during injuries. The most famous examples are Taphrina populina which causes reddish swellings on poplar leaves, but also Exobasidium rhododendri which manifests as rounded, white galls on rhododendrons. The most striking example of fungal scab is found on wild rice and is hiding under the sweet name ofUstilago esculenta which swells the stems and offers edible galls consumed in Asia.

On the bacteria side, the genus Agrobacterium full of small wonders attacking the roots or aerial parts close to the ground and causing, among other things, collar gall in many plants but also the vine brush.

This bacterium introduces a fragment of its DNA into the genome of plant cells, taking advantage of a mechanical or natural injury (lightning, frost, etc.) or a sting. Yellow / greenish galls then develop which lignify over time and increase their size sometimes dramatically, forming a circular swelling all around a trunk thus blocking the flow of sap.

Are galls dangerous for plants?

Although impressive, the galls are generally not dangerous for affected plants. When they are of animal origin (mites or insects), they never endanger the life of plants although they can somewhat disturb the phenomenon of photosynthesis if they are very numerous on the same plant, which is rare.

In this case, the plant may be weakened for a time, until the gallicolae leave their host, but unless it is extremely weakened by age, poor growing conditions or disease, the galls do not drain. enough energy for the plant to destroy it. It is a form of aliquilinism: gall is beneficial for the organism which develops there but remains neutral for the host plant.

The years are very variable and the fact of having a tree reached one day does not mean that the following year it will be, so do not panic! No need to process once the thing is apparent because the phytosanitary products will rarely reach the organisms hidden in the galls but will kill the beneficial animals.

The best treatment for leaf galls would be to remove the affected leaves, if they are not too numerous, because massive defoliation would be much more damaging than the disparate presence of a few galls.

In the case of galls caused by fungi, preventive treatments with Bordeaux mixture can help.

Regarding bacteria, compliance with optimal growing conditions but also strict hygiene rules during pruning, for example, can greatly limit the damage.

Source: Au Jardin, conseils en jardinage by

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