Sudden Oak Death
The picture above was provided by the USDA Forest Service-Region 5-FHP.
Sudden oak death is a disease of oaks and tanoaks in forests of Oregon and California (see below video). The presence of the disease is environmentally and economically damaging. P. ramorum has killed over a million trees in California in the last decade and was first observed in coastal forests of California in the mid-1990s.
Phytophthora ramorum is the microorganism responsible for sudden oak death. The organism was accidentally introduced into the US through the nursery trade. The origin of this organism remains unknown. Plants purchased from an infected nursery planted in urban oak woodland interfaces likely transmitted P. ramorum into the forest. Now, two decades later, it has spread nearly throughout the entire distribution of tanoak. P. ramorum is unusual because of its enormous host range, causing damage to more to many species of trees or shurbs. More than 100 plant species have been identified as carriers (hosts) of the organism.
Simultaneously to its introduction into the United States, P. ramorum was discovered causing dieback of rhododendrons in Europe. P. ramorum was also spread into nurseries across Europe and is currently causing epidemics in Japanese larch (Larix kaempferi) plantations in great Britain (see below video).