Abnormal Growths on Woody Ornamentals
Abnormal Growths on Woody Ornamentals
Abnormal growhts can be caused by mechanical damage, plant hormones, bacteria, fungi, insects, mites and parasitic plants.
Galls
Galls are abnormal plant growths induced by the action of bacteria, fungi, insects, mites and other organisms. When these occur on tree trunks and branches they are sometimes referred to as burls.
Galls resulting from activity of insects and mite typically develop around the gall maker, housing, protecting, and providing a steady food source for the organism within the gall. These galls may have many forms. Most insect galls, such as those produced by the gall midges, produce fairly simple galls that are the result of stunting and/or thickening of leaf tissues. These are sometimes referred to as indeterminate galls. Other insects and some mites produce very bizarre changes in the plant growth of radically different structure than the original tissues. These are called determinate galls and the various gall wasps that frequent oaks and rose are well-known for this habit. Regardless, all gall making insects and mites create galls unique for the species and they are almost invariably restricted to a single plant species.
Galls induced by pathogens and are usually restricted to woody parts of the plant. The may similarly vary in structure with many occurring as irregular growths of irregularly organized tissues. Others may produce unusual and unique forms, such as the fruiting bodies of rust fungi.
Galls can cause problems for trees when they seriously interfere with transport of food and water. However, little if any significant injury to the plant results from most galls. On the other hand the bizarre growths often attract attention and concern. Most controls are directed at preventing these aesthetic injuries.
Since only actively growing tissues are susceptible to the gall making stimuli, most galls are produced solely during the rapid growth period of late spring. Once galls are initiated, their formation is irreversible. Although the galls are often conspicuous and may be unattractive, rarely does any real plant damage occur. Under most circumstances control of galls is not recommended.
A few gall-making insects and mites overwinter on the trees and may be controlled by dormant oil applications. However, most galls are produced by insects that move to the new growth as it develops in spring and can only be controlled by insecticides that cover the leaves during the egg laying period. Because the quality of the insecticide coverage diminishes as leaves expand, repeat applications are often necessary.
Gall making insects and mites are discussed in the sections Cooley spruce gall adelgid, Pinyon spindlegall midge, Honeylocust podgall midge, Poplar twiggall fly, and Eriophyid mite galls (Poplar budgall, Ash flowergall).
Parasitic Plants Associated with Woody Ornamentals
Parasitic flowering plants that live on woody plants range from minor water robbing plants to serious parasites that derive all their nutrients and water from their host. Parasitic plants may attach to roots or the upper stems of trees.
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