| General Recommendations: Galls are swelling on plant leaves, stems, trunks, flowers or roots. They can vary in size from minute to as large as a basketball or larger in the case of some that develop on branches and trunks. Fungi, viruses, nematodes, or bacteria cause some galls but most that occur on leaves are caused by insects that stimulate the plant to produce the growth as a source of food for the insects and/or a protective home. Galls are commonly caused by some species of aphids, midges, mites and wasps. See each of these insect groupings for more information on gall forming members. Galls may also be caused by physiological factors independent of any recognized biological cause, such as burls.
Hundreds of species of cynipid wasps (family Cynipidae) produce galls, especially on oak trees. Adult gall wasps are small, stout and shiny, with purple or black bodies. They have very few wing veins. Females deposit eggs in plant tissue. Galls form several weeks or months later, after the white larvae has hatched and begun to feed. Fortunately, damage produced is not serious and does not require management.
Control of Galls:
1. Do nothing. Most insect-caused galls are curiosities and cause little damage when restricted to leaves. Galls that form on stems can cause twig dieback and give plants an unsightly appearance though rarely are a serious threat to the survival of a tree.
2. Bacterial gall can result in plant dieback and death. See Bacterial Gall for more details and specific control measures. Phomopsis gall is a common fungal gall of several plants. Some rust diseases also cause galls. Also see "Black Knot" which has a gall-like appearance. |
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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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High resolution image available.
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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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High resolution image available.
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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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High resolution image available.
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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 midge (Diptera)
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High resolution image available.
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These leafy rosettes on goldenrod (Solidago), called goldenrod bunch galls, are caused by a midge (Diptera); the plant also has rust; notice, the orange pustules on the leaf at the top of the picture
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High resolution image available.
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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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High resolution image available.
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