Landscape plants and trees often start to display small bumps on various plant areas including foliage, trunks, roots, stems and branches. These bumps are referred to as galls. Some types of galls occur in large number on affected plants while many others are rarely noticeable. Leaf galls are the most common types of galls and are seen on foliage and petioles.
Galls are the result of insects such as mites, nematodes, fungi or bacteria feeding or laying eggs on the foliage and other plant areas. This damage causes the tree to react by producing abnormally large numbers of plant growth hormones. This in turn increases the plant cell size and quantity in that area leading to the growth seen as galls. Galls are frequently seen on leaves during the active growth period of shoots and leaves.
Green Gall Identification
Galls occur in various colors depending upon the causal organism. Green gall-causing pests include adelgids, insects that are similar to aphids. Adelgids incur the most damage on conifers. The eastern spruce gall, ½ to 1 inch long, green, pineapple shaped in form, is among adelgid injuries. The galls appear on needle bases on stems. The eriophyid mites' damage causes the maple bladder gall. Pest infestation causes small, green galls on newly developing red maple and silver maple foliage uppersides. The gall gradually assumes a bright red color.
Although galls are a common occurrence, they are not a serious threat to healthy, vigorous plants. Severe infestation on stressed and weak plants may lead to defoliation and deformed growth of leaves and stems. Leaf galls do lower the aesthetic value of plants given their greater visibility and are often seen as a nuisance. It is unlikely that galls on dying trees are the cause of the decline. Galls develop only while the leaf is in its active growth stage. Once the leaf is mature and no longer growing, the galls stop appearing as well.
Since the growth of galls on leaves is the plant's response to pest infestation, any attempts to control pests after infestation has occurred will not be effective. Taking preventative measures are the best defense against galls. There is nothing to cure a gall after it has grown. Use foliar and soil-applied miticides or insecticides to kill adult pests before they damage tree. Recommended products include imidacloprid, cyfluthrin, carbaryl, spinosad and horticultural oils for use on dormant trees.
- Iowa State University Extension; Insect Galls on Trees and Shrubs; Mark Shour, Laura Jesse, Donald Lewis
- University of Minnesota Extension; Insect and Mite Galls; Robert P. Wawrzynski, Jeffrey D. Hahn, Mark E. Ascerno; 2005
- University of Illinois Extension; Leaf Galls Common on Trees; Bruce Spangenberg
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