Introduction to Phytophthora lateralis

Phytophthora lateralis is a pathogen which generally attacks and kills the roots of certain trees, although aerial infections of branches and foliage also occur.

Thought to originate in Asia, the disease is the main cause of mortality in Lawson cypress (Chamaecyparis lawsoniana) in its native range in the west coast of North America, and outbreaks have been recorded in France and the Netherlands. The pathogen was first reported in the UK in 2010, most likely having been imported from mainland Europe, and it has since been found on sites across England, Scotland, Wales and Northern Ireland. It is important that suspected cases of the disease are reported.

Infected trees show rapid decline with foliage turning a rusty-red then a dull bronze over all or a large part of the crown. The pathogen also occasionally attacks the stem or branches, causing discrete aerial infections so that the foliage turns bronze or brown in large, isolated patches in the crown. Lawson cypress is the main species of tree likely to be infected by P. lateralis in Britain. Other susceptible hosts are Sawara cypress (Chamaecyparis pisifera), Western red cedar (Thuja plicata) and white cedar (Thuja occidentalis).

However, these conifers can be easily confused with other tree species, for example, the Leyland cypress (Cupressocyparis leylandii), which is not known to be susceptible to P. lateralis.