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Wednesday, May 23, 2007 10:21 pm

Galling developments

Most are harmless, but some threaten a plant’s development

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Untitled Document While sitting on my grandparents’ porch last Sunday, I noticed numerous ball-shaped objects in their oak trees. Looking closer, I found that almost every limb of the 10-year-old trees contained gouty oak gall. A plant gall is an abnormal swelling that develops within tissues that have been colonized by a parasitic organism. The gall forms as a result of a reaction to the parasitic attack on the plant’s cells. The organism’s saliva stimulates overgrowth or cell proliferation in the host plant. The resulting structures are distinctive galls that are highly species-specific. Galls vary in size, shape, color, texture, and location on the plant. Some appear as bumps on leaves; others are large, warty growths on stems. Common galls include ash flower gall, hackberry nipple gall, maple bladder gall, maple spindle gall, gouty oak gall, horned oak gall, and oak-leaf gall. To cause a gall, an organism must feed or lay eggs on the plant while the plant parts are growing rapidly, usually in early spring. Galls may occur on leaves, stems, flowers, seeds, fruits, or roots. Many types of organisms — including nematodes, bacteria, fungi, eriophyid mites, beetles, psyllids, true bugs, wasps, aphids, and midges — cause gall. Gall-making insects and mites generally spend most of their lives feeding and growing inside the gall, which provides both a protected enclosure for the offspring and, in the inner wall of the growth, a food source. Each gall-maker is specific to the kind of plants it will attack. More than 2,000 different plant galls are caused by insects and mites, but because most galls do not cause damage resulting in economic loss, there’s been little effort devoted to their study and not much is known about these formations. Approximately 750 different galls have been identified on oak trees alone. Gouty oak gall and horned oak gall are very noticeable and noteworthy. Horned oak gall occurs on pin, black, and water oaks; gouty oak gall affects pin, scarlet, red, and black oak species. Both of these galls can be debilitating and even kill young trees. Approaches to management of these two galls are limited: If possible, trim affected twigs and branches out. Affected trees can be fertilized in mid to late fall to encourage healthy growth. Most galls do not harm a healthy established tree or shrub. Leaf galls can cause leaf distortion, and some leaves may fall prematurely. Although the galls detract from the beauty of the foliage, the infested leaves are usually able to carry out photosynthesis. Twig galls are more serious and are generally harmful to a tree or shrub. Gall formation on the twig disrupts the twig’s function, and the portion of the twig beyond the gall may die. A tree heavily infested with twig galls can over time show general decline in vigor and may occasionally die. Pruning out twig and stem galls while they are green is sometimes an effective means of control. (Small holes in the gall indicate that the inhabitants have escaped.) Because they spend such a small part of their lives outside their galls, little is known about the life cycle of many gall-makers, and so chemical control is generally not feasible. In addition, on tall trees gall-makers are hard to reach with insecticides. The timing of chemical control must be initiated before gall formation begins. Once a plant has been infested with galls, chemical sprays won’t make them go away. For more information on galls, go to Iowa State University’s “Insect Galls on Trees and Shrubs” Web page, www.extension.iastate.edu/ Publications/IC417.pdf. 

Jennifer Fishburn is a horticulture educator with the University of Illinois Extension Sangamon-Menard Unit. Contact her at www.extension.uiuc.edu/Sangamon. 
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