Chamaecyparis lawsoniana (Lawson Cypress) is an important tree species in town, suburban gardens and parks in the USA. It is economically and ecologically valuable in forests in west California and south-west Oregon. In 1854 when seed was imported from the USA to Europe for the first time, the selection and breeding of numerous cultivars started. Cultivars from Europe and additional selection from the US were propagated as ornamental plants in nurseries on the west coast of the US. In 1923 a serious root rot of Chamaecyparis lawsoniana which killed the trees was observed in nurseries and landscape plantings near Seattle (USA). The causal agent was determined as Phytophthora lateralis in 1943. In 1952 first attacks of Chamaecyparis lawsoniana trees in the forest could be observed in the USA. Today P. lateralis is widely distributed in western North America although it has not been confirmed in other parts of the continent. In Europe P. lateralis was detected for the first time on C. lawsoniana in a French nursery in 1999. There were no further findings until 2005 when the pathogen was isolated from diseased Chamaecyparis plants for planting in a nursery in the Netherlands. Since 2005 the pathogen has also been detected on several C. lawsoniana hedges in France.