How to identify plant galls

Galls are abnormal growths caused when another organism interferes with a plant’s cells. We have illustrated 12 of the most curious and colourful galls to look for.

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All illustrations by Felicity Rose Cole

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1

Bean gall (above)

Caused by the sawfly Pontania proxima. Forms on upper surfaces of willow leaves, especially crack willow and weeping willow.

2

Spruce pineapple gall

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Caused by the aphid Adelges abietis. Found on the twigs of spruce trees, often Norway spruce. Young aphids shelter inside the gall cavities.

3

Fern gall

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Made by the fly Chirosia betuleti. Occurs on ferns, especially male fern, causing rolled-over, deformed frond tips. Fly larvae dig into the fern stem.

4

Reed cigar gall

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Caused by the fly Lipara lucens. A common, cigar-shaped gall on reed stems; conspicuous in late autumn. Each gall contains a single fly larva.

5

Ergot

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Caused by the fungus Claviceps purpurea. Occurs on seeds of grass, including rye and false oat. Toxic – was a scourge of medieval Europe.

6

Tassel gall

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Caused by the psyllid (jumping plant louse) Livia juncorum. Found on flowerheads of rushes. Common in damp places throughout UK.

7

Ramshorn gall

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Made by the gall wasp Andricus aries. Occurs on oak trees, turning the buds into long prongs. Mainly in south; first seen in UK in 1997, but spreading.

8

Pea gall

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Made by the gall wasp Cynips divisa. Occurs on undersides of oak leaves, falling in autumn. Wasps emerge in winter and lay their eggs in oak buds.

9

Silk button spangle gall

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Made by the gall wasp Neuroterus numismalis. Found on oak leaves, often in big numbers. Falls to ground in autumn; wasps emerge in spring.

10

Witch’s broom

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Has many causes, including insects, mites, fungi and viruses; usual trigger is the fungus Taphrina betulina. Occurs on birch and cherry trees.

11

Nail gall

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Caused by the mite Eriophyes tiliae. Very common on lime tree foliage, appearing in summer and persisting until leaf fall in autumn.

12

Robin’s pincushion

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Made by the gall wasp Diplolepis rosae. Found on wild roses. The red, feathery mass contains many chambers, each with a wasp grub.