These defects on many garden plants are caused by various insects and pathogens. GALLS—those odd growths that appear on many plants—have intrigued people throughout the ages. They probably have been around since the first plants appeared— there isevidence of galls in fossils dating back more than 300 million years. But where do they come from and what exactly are they? Broadly defined, galls are abnormal plant tissue growths that form in response to chemical or mechanical stimuli by fungi, bacteria, viruses, nematodes, mites, or insects. Collectively, these organisms that cause galls are called cecidozoa, but we'll refer to them here as "gallers."
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