Toxicodendron diversilobum (syn. Rhus diversiloba; Western Poison-oak or Pacific Poison-oak) is a plant best known for its ability to cause allergic rashes after contact. Western Poison-oak is found only on the Pacific Coast of the United States and of Canada. It is extremely common in that region, where it is the predominant species of the genus; the closely related Atlantic Poison-oak (T. pubescens) occurs on the Atlantic Coast. The hyphenated form "Poison-oak" is used, rather than...