Leaf infections called "galls" are bumps or growths caused as a result of the feeding and other activity of insects or mites. One particularly common version of this rapid explosion of growth is called the common oak gall
and is most noticeable on the leaf, stem and twig of an oak tree. Although these galls may look like a serious problem, most are harmless to the overall health of the tree.
Galls are irregular plant growths stimulated by a reaction between plant hormones and a powerful growth regulating chemical produced by a feeding insect or mite. Galls can occur on leaves, bark, flowers, buds, acorns, or roots. The gall makers will first create the gall, then feed on the nutrients produced from the inner gall tissue. These galls also provide the insect some protection from natural enemies and insecticide sprays.
Gall makers must attack at a particular time of year to be successful. Otherwise, they may not be able to stimulate the plant to produce the tissue which forms the gall. Generally speaking, leaf galls occur around "bud break" or as new leaves begin to unfold in the spring. Unfortunately, life cycles of many gall-makers are not known so specific recommendations to time control measures most effectively are not available.
A few galls on a tree or plant seldom warrant control. They may be hand picked and discarded. Most galls are difficult to control since little is known about the gall makers and insect life cycles can vary so much. Check with your state Cooperative Extension Association horticulturist or entomologist if you have a specific gall problem.
The Gall Makers:
Insects are the major gall makers. Aphids, mites, wasps, and flies cause the majority of plant galls. In certain cases, as with cedar apple rust gall, a fungus is the primary cause.