P. ramorum Werres, De Cock, & Man in’t Veld (2001) appears to be an exotic species introduced from an unknown origin to Europe and western N. America in the mid-1990s, likely on nursery plants (Mascheretti et al. 2008). In California and southwest Oregon, the pathogen spread to native oak and tanoak forests, where it causes lethal stem cankers (sudden oak death). Between 30-45 million trees have been killed. In Europe, where nurseries in over 20 countries reported P. ramorum, the disease on trees was limited to isolated stands of beech and oak associated with infected rhododendrons. In 2009, the pathogen began causing widespread mortality (sudden larch death) on Japanese larch timber plantations in the UK (Brasier & Webber 2010). International, national, state and county quarantines are in place to prevent the further spread of disease with nursery plants and to contain the infestation in forests. P. ramorum is unusual among forest Phytophthora species for its wide host range and infection of aerial plant parts.
Sporangia (average 46–65 µm by 21–28 µm) formed on agar and in water, ellipsoid, spindle-shaped, or elongate-ovoid, semi-papillate, caducous with short pedicel (< 5 µm), produced singly or in clusters of 2–12 on sympodially branched sporangiophores. Chlamydospores (20–91 µm; average 46-60 µm) formed abundantly in agar, globose, mostly thin-walled, terminal or intercalary. Heterothallic, oogonia not seen in single culture (Werres et al. 2001).