Abstract
This chapter will focus on three key Phytophthora species (P. cinnamomi, P. ramorum and P. lateralis) that currently threaten US forests, causing widespread mortalities and changing the overall forest structure. P. cinnamomi was first recognized in the USA in the 1930s after it was isolated from dying American chestnuts and rhododendrons. The symptoms resembled those found on European chestnut trees (Castanea sativa), also known as ink disease, caused mainly by P. cambivora. P. ramorum is a relatively new pathogen first identified in the USA in 1995. It is the first known species that effectively spreads aerially in the forest setting, enabling the rapid expansion of its geographical range. P. lateralis was first recognized in the 1920s causing significant mortality to Port-Orford-cedar (Chamaecyparis lawsoniana). Its distribution spans the natural range of the Port-Orford-cedar, mainly in south-western Oregon and northern California. Other Phytophthora associations in forest ecosystems are also discussed.