Plants often develop bumps or other odd growths that might remind one of a science fiction movie, but there’s no need be alarmed. These unusual appearing growths are called galls and are usually more of a cosmetic problem rather than a health crisis.
The galls themselves are mostly made up of plant tissue, usually as an attempt to recover from insect or disease injury. Galls can be quite small, just a fraction of an inch, or can be as large as several inches long, depending on the plant and cause of injury. Some of the most common landscape plants that develop galls include oaks, maples, hackberries and roses.
Most galls occur on leaf tissue and are caused by insects. Adult insects lay eggs inside the leaf tissue, and either the adult or the developing young insects secretes a growth stimulating substance. Each insect causes a very characteristic gall. Most leaf galls are nothing to be concerned about from a plant health standpoint, although they may be unsightly. However, once the gall appears, the appearance of the current growth cannot be remedied.
Some galls, particularly those that occur on the stems of perennial plants, can be very serious problems. The galls not only disfigure the plants, but can result in eventual death of the plant. These are more likely to be caused by a fungal or bacterial pathogen. Crown gall, which affects euonymus and roses, is an example of a life-threatening gall.
Pruning out affected growth is about all that can be done once the galls appear. Pesticides to prevent insects or diseases from attacking the plants must be applied before injury occurs and the growth stimulating substances occur. For leaf gall-forming insects, insecticides must be applied during the brief week or so period that leaves unfold and fully expand in spring. Unfortunately, other pest cycles are less understood and chemical controls may prove inadequate.
Fortunately, most gall-causers are host-specific, meaning that they each have a preferred plant species. So galls that occur on maple trees, for instance, will not spread to other types of plants in the yard.
For more information on galls of ornamental plants, see the following Web sites.
Purdue publication E-56 Galls on Shade Trees
http://www.entm.purdue.edu/Entomology/ext/targets/e-series/EseriesPDF/E-56.pdf
Purdue publication BP-35 Cedar Galls
http://www.ces.purdue.edu/extmedia/BP/BP-35.html
Purdue PPDL web page – Maple Bladder Galls
http://www.ppdl.purdue.edu/ppdl/expert/Maple_bladder_galls.html
Purdue PPDL web page – Oak Hedgehog Galls
http://www.ppdl.org/dd/id/hedgehog_galls-oak.html