Phytoncide

Phytoncides are antimicrobial allelochemic volatile organic compounds derived from plants. The word, which means "exterminated by the plant", was coined in 1928 by Dr. Boris P. Tokin, a Russian biochemist from Leningrad University. He found that some plants give off very active substances that help to prevent them from rotting or being eaten by some insects and animals. 
Occurrence and use.
Cedar, garlic, locust, oak, onion, pine, tea tree, many spices, and many other plants give off phytoncides. Garlic contains allicin and diallyl disulfide. Pine contains alpha-pinene, carene, myrcene, and other terpenes. "Sophora flavescens" contains sophoraflavanone G. 
More than 5,000 volatile substances defend plants that protect them from bacteria, fungi, and insects. Phytoncides work by inhibiting or preventing the growth of the attacking organism.
They are widely used in Russian, Ukrainian, Korean, Chinese, and Japanese medicine, as well as in holistic medicine, aromatherapy, and veterinary medicine.