Leaf infections called "galls" are bumps or growths caused as a result of the feeding and other activity of insects or mites. Powerful growth regulating chemicals produced by feeding insects or mites cause galls can occur on leaves, bark, flowers, buds, acorns, or roots.
Oak Apple Gall
Steven Katovich, USDA Forest Service, Bugwood.org
Oak apple gall is a round, one to two inches in diameter growth with a spongy inside and a hard central core. These galls are seen on the leaves of many oaks. They are caused by a tiny wasp and are usually seen in May or June.
Maple Bladder Gall
William H. See/About.com Forestry
Maple bladder gall is caused by mites. The tiny growths are about 1/8 inch in size, with most of the swellings occurring on the upper leaf surfaces. The newly formed galls are a yellowish-green color. Towards the end of June they turn reddish and late in the season they are black.
Gouty Oak Gall
Gerald J. Lenhard, Bugwood.org
Gouty oak gall is caused by a wasp. These galls appear on the stems of black, red, pin, and scarlet oaks. They have been known to cause small branches to be killed and break off the tree.
Maple Spindle Gall
Ohio State University Extension Photo
Maple spindle gall is caused by mites. The galls are found on soft maples and sugar maples. The galls are about 1/5 inch long and about as thick as pencil lead. The galls stand erect and are easily seen and identified.
Maple Eyespot Gall
Steven Katovich, USDA Forest Service, Bugwood.org
The maple eyespot gall is caused by a small midge. The fly lays its eggs in the under surface of the leaves. The maggot that hatches develops quickly and produces a growth regulating hormone that causes the bright red and yellow rings around the gall, usually in June.
Ash Midrib Gall
Laura Lazarus, North Carolina Division of Forest Resources, Bugwood.org
Ash midrib gall is found in the center of the leaf along the midrib of white ash. This gall may be up to an inch in length and is plump and spindle-shaped. It is caused by a tiny fly called a midge.
Cedar Apple Rust
Joseph O'Brien, USDA Forest Service, Bugwood.org
Cedar apple rust on junipers (including redcedar) is a tan to brownish fungal gall and is not caused by an insect. With moist weather, gaudy bright orange masses of gelatinous spores develop from these galls.
