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Untitled-2Damage Type

Abnormal growths clearly visible on leaf parts/stems; species specific varying with agent and host though commonly bulbous, raised and egg-like in appearance. Often a different colour to infected/affected foliage/sections.


A gall is an abnormal growth reaction of the plant in response to an intrusive organism such as a bacterium, fungus, nematode (eel worm), mite, insect or possibly a virus. Galls form when plant tissue cells become enlargened or multiply excessively as a direct result of complex bio-chemical processes. Gall-causing organisms are parasitic, providing the stimulus for the formation of the gall but does not make it, it is purely a catalyst for the creation, the agent living inside or outside of the tissue. Most galls are of a characteristic form typical of the causal organism, usually of a constant size, though again some may vary greatly. The majorities are of an annual appearance on the softer leaf tissue, those occurring on woody stems, roots and even buds may persist for several years.

Symptoms & Diagnosis

There are a wide variety of forms: many are protuberances of a characteristic shape within which the causer feeds and reproduces; others are composed of dense patches of hair usually on the underside of the leaf known as erinium galls. The gall may take the form of a rolled and maybe swollen edge of the leaf. Some galls appear visually spectacular forming where a proliferation of axillary buds lead to the formation of a ‘witches broom’ growth many of which are caused by fungi or other microorganisms.


The majority of galls present no risk to the health of the tree, affecting only the appearance of the foliage but a cause for no real concern.

Host Trees

Most native genera of broad-leaved and coniferous trees are affected by galls especially on the leaves.


Rarely necessary and not usually feasible on large mature trees. In a few special cases damage is of some concern and in extreme cases may prompt some degree of control.

Types: (foliage)

  • Nail & bead galls: elongated or rounded protuberances on the upper surface of the leaf. Colour is sometimes different to the leaf itself, the galls being smooth or hairy. Most often these are induced by eriophyid mites, gall midges or aphids. Nail galls occur commonly on Acer (maple), Fagus (Beech) and Tilia (lime). More globular bead galls are found on Acer (maple), Populus (Poplar) and Ulmus (Elm). Bean-like swellings on the foliage of Salix (willow) are galls caused by saw-flies.
  • Blister Galls: Exhibited as large greenish blisters on the leaf up to 15mm or more in diameter occurring on the upper side of Walnut leaflets. The underside of these galls is lined with whit hairs among which live microscopic eriophyid mites, the causal agent. Much smaller mite-induced blister galls causing only a roughening of the leaf occur on Sorbus (Whitebeam). Similar species also affect Ulmus (Elm) and Acer (maple). Large blister galls on Prunus species are caused by fungus.
  • Leaf margin rolled or swollen: The edge of the leaf is tightly rolled forming a continuous piping, sometimes forming a more irregular folded edge. Galls of this type are caused by aphids or gall midges (Acer (Maple), Malus (apple), Pyrus, Prunus, Quercus (Oak), Salix (willow) , Tilia (lime) and Ulmus (elm), eriophyid mites (Crataegus (Hawthorn), Fagus (Beech), Salix & Tilia (lime)) and psyllidsor suckers (Fraxinus (Ash), Laurus (Laurel)).
  • Erineum or felt-galls: Felt-like or velevety patches of short, dense hairs, induced by eriophyid mites on either the upper or underside of the leaf. Initially the same colour as the leaf darkening and often becoming brown as the season progresses. Known to occur on Acer , Aesculus (Horsechestnut), Alnus (Alder), Betula (Birch), Crataegus, Fagus, Fraxinus, Malus, Populus, Sorbus aucuparia (Rowan/Mountain Ash) & Tilia.
  • Cynipid Wasp Galls: Galls of many different shapes and sizes are caused by these insects on Oak, most familiar and common being the Common spangle gall found on the underside of the leaves in late summer, and the larger cherry gall on the upper surface in late spring and early summer.
  • Petiole: Swollen and distorted petioles result mainly from the activity of aphids. Typically on poplar are the purse galls and spiral galls causing little or no harm to the host; however the alternate form of the aphid migrates to the roots of thistle which can then cause serious damage to commercially grown crops.

Bud Galls

  • Enlarged buds that do not flush with the branchlet/stem due to the activity of the eriophyid mites occur on Betula (Birch) and Corylus (Hazel).
  • Oak buds may be damaged by a variety of cynipids that cause change of shape. Common are the artichoke gall; this has many overlapping scales around a central core containing the gall wasp larvae.

Shoot Galls

On Picea (Spruce) these are partly swollen shoots, pineapple-like in appearance when the needles are shortened as they often are. These galas appear from May onwards. By August or early September they are mature and openings develop through which the causal agent (adelgids) escapes. They are purely disfiguring and do not cause harm to the host, and may be picked when apparent to negate poor aesthetics.


Swelling and fusion of the pedicels and peduncles forming brownish lumpy masses caused by a mite appearing in spring and remaining conspicuous on the tree throughout the year. Currant galls (cynipid) are common on Oak flowers. Mossy catkin galls occur on willow, these are witches’ brooms caused by the transformation of the flowers into bunches of vegetative shoots.

Fruits & Seeds

The only common example is the Knopper gall on the acorn cup of the English Oak widespread in England & Wales and spreading in Scotland.

Stem Galls

Swellings on twigs occurring on Poplar and willow due to the activity of a long-horn beetle or weevil.

Crown Galls

Typified by one or a number of roughened warty more or less spherical galls on the stem at ground level. This is caused by the bacterium Agrobacterium tumefaciens persisting in the soil and entering the plant through wound abrasions. The tiny young galls are soft and whitish becoming hard with age and assuming the colour of the tissue from which they grow. They can reach the size of a football depending on the species of host! Galls may develop on roots and sometimes (often in rows) on the stem higher up. If the galls encircle a stem the young tree may be stunted and flowering/fruiting impaired, but often no harmful effect is apparent.