Phytophthora ramorum .(P. ramorum) is a water mold pathogen that causes Sudden Oak Death (SOD). It also causes the Ramorum Leaf Blight, Ramorum Dieback and Phytophthora Canker Diseases.

P. ramorum produces bark cankers, leaf spots and twig dieback. Cankered trees may survive several years. In the final disease stage, the leaves turn from green to brown within a few weeks. Black or reddish ooze often seeps from the cankers. The cankers gradually expand until they girdle the tree, causing it to die. Other diseases cause similar symptoms, so laboratory confirmation is needed for accurate diagnosis.

P. ramorum effects a broad range of trees and plants of which there is no known cure, including hardwoods, softwoods, landscape plants and herbaceous plants. To date, more than 75 plant species and cultivars representing more than 45 genera can either be infected by P. ramorum or facilitate its spread.