CSU Extension Galls are abnormal growths of plant tissue produced in response to a wound, infection by various microorganisms, or by the feeding and egg laying activity of certain insects and mites. Galls are known to occur from almost any plant tissues but the most commonly observed are leaf, stem and flower galls produced by insects and mites.
Galls often are unusual in form, very conspicuous and frequently cause considerable concern.
However, plant galls produced by insects and mites uncommonly cause serious threat to plant health. Most gall-maker populations fluctuate from season to season and the occurrence of many galls in one year usually is followed by few the next year.