Plant Galls

Galls are abnormal, vegetative growths that are usually formed as a response by plants to the action of fungus, mites, or insects such as wasps, aphids, and true bugs. Galls can be formed in the leaves, petioles (stem) of leaves, twigs, buds, or on the roots. 

Oak Gall Combo

Several species of gall wasps are responsible for the formation of the leaf galls known as "oak apple galls." Oak apple galls have a firm to hard outer shell when dried (far left photo) and are filled with a fibrous or spongy material, and may be attached to the tree by the leaf petiole or may be formed on a seemingly normal leaf. These galls appear in the spring (right photo) and may remain on the tree into the fall. 

 The causative agent of the oak apple gall shown above is a parasitic wasp, Biorhiza pallida. The fibrous strands filling the inside of this gall are visible in the photo on the right.

Oak Gall Inside
Oak Gall Before

The thin-shelled gall pictured on the left was caused by the gall wasp Loxaulus maculipennis.

 In the image on the right, the gall has been cross-sectioned to reveal the contents of a small amount of fibrous material radiating outward from a central point inside the shell.

Oak Gall After
Woolly Oak Combo

Wool-sower galls (far left photo) are 1 1/2 inch to 2 inch long woolly twig galls with reddish seed-like grains. Woolly oak leaf galls (pictured right) are similar in appearance to a dense wad of light brown wool attached to the midvein of the leaf (although occasionally are found on the lateral veins). Both wool-sower and woolly oak leaf galls are caused by parasitic gall wasps belonging to the genus Callirhytis.  

The white, woolly twig gall in the photo on the left is caused by Andricus furnessae, a species of gall wasp. Another species of gall wasp, Plagiotrochus punctatus, is the causative agent for the gouty oak gall (pictured right), which occurs on twigs.  

White Fuzzy Combo
White Oak Stem Gall

Eastern oak bullet galls (pictured above) are smooth, spherical galls approximately 1/2 inch in diameter produced by a wasp of the genus Disholcaspis. These galls are very hard because they form from the tough, woody tissues of twigs.  

The large swellings, such as the one on this oak, are caused by either bacterial or viral infections. The growth is similar to a tumor and can be as large as a meter (over three feet) wide.  

Bacteria Gall Oak
Hickory Nipple Galls

The small galls on the underside of the hickory leaflet pictured on the left are caused by Phylloxera caryaefallax, a species of plant louse.  

The globular hickory leaf galls in the top photo are produced by the plant lice Phylloxera caryaeglobuli. The causative agent of the hairy, globular leaf galls (shown in photograph on the bottom) are gall midges belonging to the genus Caryomyia

Hickory Gall Combo
Elm Combo

The globular leaf galls (top) and the elm finger galls (bottom) were caused by a species of mite belonging to the genus Eriophyes

The globose, red-tinted pocket galls on the hop-hornbeam leaf pictured here were caused by tiny (1/25 - 1/12 inch) mites belonging to the genus Eriophyes

Hop-Hornbeam Gall
Cypress Combo

The cypress twig gall midge, Taxodiomya cupressiananassa, causes swellings on bald cypress branches with some leaves included in the galls.  

A fungus called Gymnosporangium juniperi-virginianae causes this unsightly cedar apple rust gall to appear on eastern redcedar trees (alternate host). The fungus overwinters inside these galls, then the orange jellylike "horns" (called telia) seen in the above photograph grow out of the gall in the spring and produce spores which may infect apple leaves to continue the life history of the fungus.  

Cedar Apple Gall Wet
Hackberry Gall

Hackberry petiole galls (see swelling at top of lower leaf) are caused by jumping plant lice of the genus Pachypsylla, which resemble tiny cicadas. Nymphs overwinter in the galls, and the dead leaves tend to remain on the trees until autumn.  

 The dogwood club gall is not as obvious as most galls, appearing as a brownish swelling at the end of what normally would be a green stem (right image). This gall is caused by Mycodiplosis alternata, a species of gall midge (a fly).

Dogwood Gall
Blueberry Stem Gall

The gall wasp Hemadas nubilipennis causes kidney-shaped galls with a diameter of approximately 1/2 to 1 inch to form on blueberry stems (pictured left).  

Round or elongated (2 to 6 inches in length) blackberry stem galls with deep, longitudinal furrows are caused by the gall wasp, Diastrophus nebulosus. These stem galls are known as blackberry knot galls (pictured right).  

Blackberry Gall
Poison Ivy

These puckered, irregular, wart-like galls on poison ivy are caused by an Eriophyid mite, Aculops toxicophagus.  

The Goldenrod Gall Fly (Eurosta solidaginis) lays its eggs in the stem of the goldenrod. The plant reacts to the larva's chewing at the affected site by swelling into a ball (left photo). This phenomenon creates a "goldenrod gall." Notice the small hole near the base of the gall. This is the hole from which the fly exits after maturing. The spindle-shaped gall in the photo on the right is made by the larval stage of a moth (Gnoremoschema gallaesolidaginis).  

Goldenrod Combo
Black Gum Leaf Curl

 The leaves of black gum (Nyssa sylvatica, seen here) sometimes show folding or curling of leaf margins due to either midges (Cecidomyia nyssaecola) or mites (Eriophyes sp.).

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