Phytophthora ramorum

A newly isolated pathogen responsible for severe oak mortality on the west coast of the U.S.

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Name: Phytophthora ramorum
Taxonomic Position:
Chromista: Oomycota: Oomycetes: Pythiales: Pythiaceae
Common Names: Causal agent of Sudden Oak Death

This fungus was described recently, and is the pathogen responsible for the rapid death of thousands of oaks along the California coast.

Issues of Concern: First observed in 1995, Sudden Oak Death (SOD) has achieved epidemic proportions. Trees are affected in urban and rural forests as well as private landscapes and public parks. In some counties, approximately 40% of oak and tanoak trees are dying. In addition to economic losses, the abundance of dead trees increases the risk of fires. There is concern this syndrome will spread further due to the presence of oak host-dominated communities throughout the coastal counties of California and Oregon.

In Europe, SOD was first found on nursery plants and Rhododendron plantings southwest of England. In November 2003, one Southern red oak tree (Quercus falcata) was found infected with P. ramorum and in the vicinity of the previously infected Rhododendrons (see News Story). In December 2003, new hosts, including European beech (Fagus sylvatica), horse chestnut (Aesculus hippocastanum), and holm oak or holly oak (Quercus ilex, were found infected with SOD adjacent to an earlier site of infection on rhododendron. (For more information, visit http://www.defra.gov.uk/news/2003/031204c.htm.)

There are two mating types: A1 (originally only in Europe) and A2 (originally only in the United States). Recently, the A1 mating type was found in the United States, and the A2 mating type was discovered in Europe (see News Story).

Hosts: Tanoak (Lithocarpus densiflorus), Coast live oak (Quercus agrifolia), Black oak (Quercus kelloggii), and Shreve oak (Quercus parvula var. shrevei).

Other regulated hosts (not all of which die following infection) include Arrowwood (Vibernum x bondnantense), Bay laurel (Umbellularia californica), Bigleaf maple (Acer macrophyllum), Buckeye (Aesculus californica), California coffeeberry (Rhamnus californica), California honeysuckle (Lonicera hispidula), Huckleberry (Vaccinium ovatum), Madrone (Arbutus menziesii), Manzanita (Arctostaphylos spp.), Rhododendron (Rhododendron spp.), and Toyon (Heteromeles arbutifolia)

More recently the following hosts have been added: Douglas fir (Pseudotsuga menziesii var menziesii), Coast redwood (Sequoia sempervirens), Andromeda (Pieris formosa), Laurustinus (Viburnum tirus), Camellia (Camellia japonioca), Western star flower (Trientalis latifolia), Canyon live oak (Quercus chrysolepis), European beech (Fagus sylvatica), Horse chestnut (Aesculus hippocastanum), and Holm oak (Quercus ilex).

Vector(s)/Dispersal: Ambrosia beetle (Monarthrum scutellare and M. dentiger) and western oak bark beetle (Pseudopityophthorus pubipennis) infestations are common in advanced Sudden Oak Decline, but have not yet been demonstrated as vectors.

U.S.A.: California (Mendocino, Sonoma, Napa, Marin, Solano, Alameda, Santa Clara, San Mateo, Santa Cruz, Contra Costa, Humboldt, and Monterey Counties); Oregon (Curry County)

Also present in Europe in seven countries.

Quarantines: See Federal Register, Vol. 67, No. 31 (Thursday, Feb. 14, 2002): accessible through the USDA, APHIS, PPQ, Invasive Species and Pest Management Website http://www.aphis.usda.gov/ppq/ispm/sod/ (check this website for updates to regulated host list).

Detection Strategies
Initial symptoms include wilting shoots throughout the crown and paling of older leaves. Leaves subsequently turn brown (within 2-3 weeks) but remain attached to the tree. Advanced Sudden Oak Death is characterized by the presence of burgundy-red sap seeping from the lower trunk (bleeding). Discolored tissue is found beneath the bark, and in extreme cases will extend into the wood.

The nomenclature used is based on Ainsworth and Bixby's Dictionary of the Fungi. The website links below provide recent information and maps.

Useful Links:
Marin County Oak Project
California Oak Mortality Task Force (COMTF) Website
Rhododendron connection
Oregon Department of Agriculture
Department for Environment Food and Rural Affairs (defra)
Canadian Food Inspection Agency (CFIA) Sudden Oak Death Fact Sheet
NAPIS Sudden Oak Death Site

Phytosanitary Alert System
Pest Alert www.pestalert.org
Prepared on: 12/22/2003