Sudden oak death and ramorum disease: what the agencies record

This page reports what the official source says. It is not legal advice: ask the office named here. It lists what agencies record about the water mould Phytophthora ramorum and does not diagnose any tree. It causes sudden oak death on oaks and tanoak in the western United States, and ramorum disease on larch and other trees in the United Kingdom. The list of other diseases is on the tree diseases page.

At a glance

Item	What the sources say
Pathogen	Phytophthora ramorum, which Forest Research calls an algae-like water mould. The California Oak Mortality Task Force calls it a fungus-like plant pathogen.
Hosts	Forest Research says more than 150 plant species. The Task Force says well over 100, including hardwood and conifer trees, shrubs and ferns.
United Kingdom	Present in most regions, more often reported in the wetter west. Forest Research lists it as notifiable. Larch is the main tree host named.
United States	The Task Force says it was first recognized on oaks in California in the mid 1990s and that federal quarantine rules are made by the Animal and Plant Health Inspection Service, with extra rules in California counties and part of Oregon that have wildland infestations.
Canada	The Canadian Food Inspection Agency says it has detected the pathogen on host plants in nurseries in southern coastal British Columbia since 2003.
Report to	See the report-a-pest guides for the United Kingdom, United States and Canada.

Which trees the sources name

Forest Research says larch is the main forest host in the UK, and that European sweet chestnut has been found affected in southern England since 2015. Beech, southern beech, some non-native oaks such as red oak, Turkey oak and holm oak, and horse chestnut can be occasional hosts, and Douglas fir, grand fir, noble fir and western hemlock can be infected when growing near infected larch. It says the genetic forms in the UK have had little effect on the two native oaks, pedunculate and sessile, while different forms in the United States have caused significant damage to North American oaks and tanoak. The Task Force says infection has been found only in oaks of the red or intermediate groups, that oaks in the white group are not susceptible, and that tanoak is the most susceptible known host.

How the sources say it spreads

Forest Research says spores can travel several miles in mist, air currents, watercourses and rain splash, and that they also move on footwear, tools, equipment and vehicle wheels. Movement of infected plants is a key way it spreads over long distances. Rhododendron ponticum and infected larch are named as spore producers: infected larch release many more spores than rhododendron. Scottish Forestry says mud and needles stuck to footwear, machines, vehicles and bicycle or buggy tyres can carry it. The Task Force says the pathogen was inadvertently introduced to California forests on nursery stock, and that California bay laurel is the most important foliar host that spreads spores, and that it has not seen bay laurel dying from the disease.

Symptoms as the sources state them

Larch and conifers. Forest Research lists bleeding cankers that ooze dark fluid from the bark, which can dry to a crust, with discolored dying inner bark. It lists wilted shoot tips with blackened needles that fall early, and says trees die when lesions become extensive on the main trunk.
Oaks and tanoak. The Task Force describes cankers on the trunk with red-brown to black discoloration that seep dark sap, usually 1 to 2 meters up, on larger trees only. It says the name came from browning of the whole crown in 2 to 4 weeks, but that death usually follows an extended infection of more than two years, during which outside signs may be slight or absent.
Shrubs. Both sources describe leaf spots and shoot dieback on rhododendron, camellia and similar plants. The Task Force says leaf and twig symptoms kill only very small plants.
Season. Forest Research refers to spring and autumn periods when spores form on larch needles.

Look-alikes and lab confirmation

The Task Force says diagnosis can be difficult because other fungi, insects and poor growing conditions produce closely matching symptoms, and that the only way to confirm infection is to take a sample and analyze the tissue in a laboratory. Its sampling page says bark sampling is invasive and needs equipment from the laboratory, and names a University of California Cooperative Extension pest alert with a section on look-alike conditions. Scottish Forestry says suspected sites are followed up with ground surveys that collect samples for molecular diagnosis at Forest Research, and that reports of suspected regulated pests go to its Tree Health Officers. Forest Research says suspected cases on trees or other woodland plants must be reported to the forestry authorities.

Rules the agencies describe

Forest Research says the Forestry Commission, Scottish Forestry, Natural Resources Wales and Northern Ireland's agriculture department serve Statutory Plant Health Notices on owners or managers of affected woodland, requiring them to destroy infected and nearby trees, and that failure to comply can lead to enforcement action. Scottish Forestry says it flies aerial surveillance of larch every year and publishes an outbreak map and a notice dataset. The Canadian Food Inspection Agency says a nursery with a detection goes under quarantine and its infected plant material is destroyed. Its page, last modified February 11, 2022, lists regulated areas, a list of regulated plants and a nursery response protocol.

What the agencies list for management

Forest Research says no cure has been found and no effective chemical treatments are available, and that some fungicides suppress symptoms but none kills the pathogen. Its stated aim is to prevent or slow spread, and it says infected, spore-producing plants such as larch should be felled or otherwise killed as soon as possible after detection, ideally before spores start forming again on the needles in spring or autumn. This is called sanitation felling. For who may apply pesticides see the guide to pesticide licences.

Credit

The Forest Research and Scottish Forestry material was read under the Open Government Licence v3.0 and is restated here in our own words. Other pages are linked and restated, not copied.