Galls are abnormal growths on plants that can be caused by a number of different organisms including insects and diseases. Following are some of common galls. Most are found on trees and shrubs but other plants can also develop galls.

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Burl on oak (Quercus)
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Azalea gall caused by the fungus Exobasidium vaccinii, shown here on azalea (Rhododendron), can also infect plants in the genus Vaccinium
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Azalea gall caused by the fungus Exobasidium vaccinii can infect the leaves, branch tips, flower parts and even seed pods of its host; here it is on azalea (Rhododendron)
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Azalea galls (caused by the fungus Exobasidium vaccinii) should be pruned out before developing the whitish coating indicating the presence of spore producing bodies as shown here on azalea (Rhododendron)
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These leafy rosettes on goldenrod (Solidago), called goldenrod bunch galls, are caused by a tephritid fruit fly (Diptera--Procecidochares atra))
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Close-up of a leafy rosette, called a goldenrod bunch gall, on goldenrod (Solidago); these are caused by the feeding of a tephritid fruit fly (Diptera)
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These leafy rosettes on goldenrod (Solidago), called goldenrod bunch galls, are caused by atephritid fruit fly (Diptera); the plant also has rust; notice, the orange pustules on the leaf at the top of the picture
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Rosette gall caused by the midge Rhopalomyia solidagninis (Diptera) on Canada goldenrod (Solidago altissima) with an unknown rust
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White blister galls on goldenrod (Solidago) caused by a midge (Diptera), Asteromyia carbonifera
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White blister galls on goldenrod (Solidago) caused by a midge (Diptera), Asteromyia carbonifera
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Close-up of white blister gall on goldenrod (Solidago) caused by a midge (Diptera), Asteromyia carbonifera
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Blackberry seed gall on blackberry (Rubus) caused by a gall wasp (Hymenoptera) Diastrophus cuscutae-formis
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Blackberry seed gall on blackberry (Rubus) caused by a cynpid gall wasp (Hymenoptera) Diastrophus cuscutae-formis
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Blackberry seed gall on blackberry (Rubus) caused by a gall wasp (Hymenoptera) Diastrophus cuscutae-formis
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The swelling on this dusty miller (Senecio) could easily be mistaken for a flower bud but is really a gall caused by a midge (Diptera)
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Bud gall on artemisia (Artemisia) caused by a midge (Diptera)
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Cross-section of bud gall on artemisia (Artemisia); note, galleries and orange larva of midge larvae that caused the gall (Diptera)
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Kidney-shaped stem gall on blueberry (Vaccinium) caused by a wasp (Hymenoptera)
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Cross-section of stem gall on blueberry (Vaccinium); note, galleries of wasp larvae (Hymenoptera)
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Leaf gall on poison ivy (Toxicodendron radicans)
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Azalea twig gall caused by the fungus Exobasidium vaccinii on azalea (Rhododendron)
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Azalea twig gall caused by the fungus Exobasidium vaccinii on azalea (Rhododendron)
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Flower bud gall on rudbeckia (Rudbeckia laciniata), possibly caused by a midge (Asphondylia rudbeckiaeconspicua)
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Flower bud gall on rudbeckia (Rudbeckia laciniata), possibly caused by a midge (Asphondylia rudbeckiaeconspicua)
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Dissected flower bud gall on rudbeckia (Rudbeckia laciniata), possibly caused by a midge (Asphondylia rudbeckiaeconspicua)
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Flower bud gall on rudbeckia (Rudbeckia laciniata), possibly caused by a midge (Asphondylia rudbeckiaeconspicua)