8/20/01

Sudden Oak Death in Southwest Oregon

Mortality of tanoak in Southwest Oregon caused by Phytophthora ramorum. Photos by E. Goheen

A pathogen that has killed thousands of oaks in coastal central California has recently been found in southwestern Oregon. Are we concerned? Absolutely!

What is Sudden Oak Death?
What is now called Sudden Oak Death or "SOD" was first reported in 1995 in Mill Valley, California. Residents of the urban-forest interface observed rather striking and seemingly spontaneous death of tanoaks in the forests close to their homes.

Coast live oak dying from Sudden Oak Death in a mixed-evergreen forest in Marin County. Photo. P. Svihra, UCCE, Marin County.

Since that time, large numbers of tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), and black oak (Quercus kelloggii) have died in the forests of Marin, Mendocino, Monterey, Napa, Sonoma, Santa Clara, San Mateo, and Santa Cruz counties. Up to 80 percent of the oaks in some stands have been killed. The causal agent, Phytophthora ramorum, a previously undescribed species, also kills Shreve's oak (Q. parvula var shrevei), Pacific madrone (Arbutus menziesii) and evergreen huckleberry (Vaccinium ovatum). P. ramorum has been found to cause tip dieback and leaf spotting of myrtlewood (also known as California bay-laurel) (Umbellularia californica), Pacific madrone, California buckeye (Aesculus californica) and rhododendron cultivars (Rhododendron sp.) in California and European nurseries.

Dying tanoaks in a redwood-tanoak forest in Marin County. Photo by Pavel Svihra, UCCE Marin County.

The extent of Sudden Oak Death is not fully known and much needs to be learned about the biology and ecology of P. ramorum. Results from studies that are underway may not be available for some time. P. ramorum is new to science and many of the processes being studied are relatively slow. Experts in California suggest that if widespread mortality continues or accelerates and the area affected increases significantly, the environmental consequences will be devastating. The affected trees and shrubs are important to urban and wildland forest ecosystems. They are essential components of healthy watersheds, provide wildlife habitat and food, and are important both aesthetically and as commodities. Not only that, acres of dead, dry oaks will substantially increase fire hazard in the already fire-prone ecosystems of central California.

What is Phytophthora?
The genus Phytophthora is included in a group of microorganisms known as the "water molds". As the name implies, this group is ecologically favored by the presence of free water in soil and on foliage. Phytophthora species have the capability of movement in water via spores that can swim short distances. Phytophthora species can remain viable in soil organic material for several years by producing thick-walled, drought-resistant resting spores. Some species have fruiting structures that are capable of aerial dispersal and spread. For years Phytophthora was included with the fungi, however plant pathologists have long recognized that Phytophthora is unique and has many special features that make its classification in the fungal kingdom a less than perfect fit. Phytophthora is currently taxonomically classified with organisms such as the brown algae. Nonetheless, it is considered "fungal-like" and is still often referred to as a fungus.

Species in the genus Phytophthora are mainly, if not all, parasitic on a variety of host plants. Diseases caused by Phytophthora species have a great and well-deserved reputation for spreading rapidly and far, for causing severe damage, and for being extremely difficult to control. In fact, the Latin translation of the word Phytophthora is "plant-destroyer." Late blight of potato which destroyed Ireland's potato crop in 1845 and 1846, Jarra decline, which causes widespread mortality of eucalypts and other plant species in Australia, and Port-Orford-cedar root disease, familiar to us in southwestern Oregon and northwestern California for mortality in Port-Orford-cedar, are all caused by Phytophthora species.

Where Phytophthora ramorum came from is unknown. This new species has never been seen before in the United States. The only other places it has been reported are the Netherlands and Germany; where it has been found on rhododendrons and Viburnum in landscape and nursery settings

Biology and Spread of Phytophthora ramorum
P. ramorum has been recovered from diseased plant material, from woody material that has been on the forest floor for several months, from leaf litter, from soil, and from rainwater captured in the vicinity of diseased trees. Its spread biology is not completely understood; however, evidence strongly suggests that it may be transferred in rain splash as well as in soil and plant material that is moved from place to place. In the laboratory, P. ramorum is favored by relatively cool conditions; optimum growth in culture occurs near 20 deg C (68 deg F) and growth is negligible at 30 deg C (86 deg F). At lower temperatures and in the presence of free moisture spores are released that can swim through water films to seek out suitable infection sites.

Investigations thus far suggest that P. ramorum invades tree hosts through the bark on stems and limbs. Oozing cankers result. Phloem tissue and the outermost layers of the xylem are killed. Vigor decreases and the tree becomes vulnerable to other organisms including bark beetles and ambrosia beetles. Bark beetles feed in the remaining intact inner bark and ambrosia beetles bore into the wood. These insects normally do not colonize healthy, vigorous trees but can be very destructive when trees are weakened. Other fungi, including a frequent associate, Hypoxylon thouarsianum, may also colonize the wood and bark of weakened trees, contributing to their demise and increasing the difficulty of retrieving and identifying P. ramorum.

P. ramorum was also recently isolated from the leaves of tanoak, previously it had only been known to infect the inner bark and a small portion of the outer wood on stems and branches. Researchers believe infection on tanoak leaves, along with increased susceptibility, may be a contributing factor in the larger number of dead tanoaks. Similar to what is found on Pacific madrone, myrtlewood, evergreen huckleberry, and rhododendron, foliar infections may allow for the rapid build-up of P. ramorum in the environment. The infections may serve as a reservoir for inoculum increasing the risk of infection of nearby susceptible oaks and tanoak.

Symptoms of Sudden Oak Death Symptoms of Sudden Oak Death vary slightly depending upon the plant species affected. On diseased trees, they include: burgundy red to tar black sap bleeding on the bark surface, sunken or flattened areas (cankers) beneath the bleeding, and distinctive black zone lines between diseased and healthy tissue in the inner bark of the trees. Cankers and bleeding occur above ground, usually from 1 to 15 feet above the soil line. Bleeding is not always observed on infected tanoaks, particularly not with smaller diameter stems. In addition, the new growth of tanoaks may wilt or droop before any bleeding appears. P. ramorum causes tip and branch dieback of evergreen huckleberry; in some cases plants have been killed down to the groundline. It causes tip dieback and leafspotting of myrtlewood, Pacific madrone, and rhododendron cultivars. It has been observed killing Pacific madrone but the extent of mortality in madrone is not understood at this time. Many other agents cause cankers, wilting dieback, and leafspots on all SOD hosts. It is very difficult to diagnose the disease in the field and in all cases, culturing or molecular testing for the P. ramorum is required before positive identification can be made. Many individuals and agencies are involved in trying to better understand and manage SOD. In California, plant pathologists, entomologists, ecologists, arborists, watershed managers, recreation specialists, and California state and local politicians, among others, have teamed up to coordinate surveys, research, management strategies, public education and funding related to SOD. The California Oak Mortality Task Force has been assembled to coordinate these efforts and to assist communities affected and threatened by SOD to maintain a safe and healthy environment. The task force maintains an excellent website, www.suddenoakdeath.org, providing the latest information on SOD and linking to recent research findings coming from the University of California at Davis and Berkeley.

Sprouts at base of dead tanoak. Photo by Joseph O'Brien. USDA Forest Service Cankers on diseased tanoak. The healthy tissue (light color) is separated from dead tissue (dark color) by dark brown-black line. Photo by E. Goheen, USDA Forest Service. Leaf symptoms on bay laurel (myrtlewood) in California caused by P. ramorum. Photo by Joe O'Brien, USDA Forest Service. The most distinctive symptom of P. ramorum on huckleberry is a canker (darkened area with dead tissue underneath) on the cane. Photo by Dave Rizzo, UC Davis.

Burgundy red to tar black sap bleeding on the bark surface of a diseased tanoak. Photo by E. Goheen	Ambrosia beetle frass at the base of a diseased tanoak. Photo by E. Goheen.

The Situation in Southwest Oregon
In addition to ground surveys begun in 2000 as a cooperative effort by the Oregon Department of Agriculture, Oregon Department of Forestry, and the USDA Forest Service, a special aerial detection survey for SOD was done in Southwest Oregon in late July 2001. The area surveyed was selected based on influence of coastal environmental conditions, high proportion of tanoak in the forest, and proximity to heavily used transportation routes between California and Oregon. Patches of dead tanoak were mapped and later visited and evaluated on site. Isolations were made onto Phytophthora-selective media and cultures that grew from isolations were checked against known P. ramorum cultures and P. ramorum DNA. Four sites, all located approximately two to three air-miles northeast of Brookings, Oregon, have been confirmed as having Sudden Oak Death. The sites include federal, private industrial, and private nonindustrial forestlands. Sites range from less than one acre to approximately 8 acres in size. Tanoak, evergreen huckleberry, and Pacific rhododendron (Rhododendron macrophyllum) appear affected.

Sudden Oak Death is a Threat to Southwest Oregon's Plants and Forests
Southwest Oregon has the hosts, the climatic conditions preferred by the pathogen, and many potential pathways for its movement; chances of continued introductions and establishment appear high. If established, the SOD Phytophthora would pose an especially great hazard to host ecosystem components in Southwest Oregon.

All of the plant species reported as hosts of the SOD Phytophthora, except coast live oak, Shreve's oak, and California buckeye, are important species in our area. Black oak and tanoak are key ecological components of oak woodland and coast range ecosystems in southern Oregon. Tanoak grows in mixed conifer stands on the moist slopes of the coastal mountains. It can grow on a variety of soils including serpentine soils. It also forms pure stands. Tanoak leaves and branches are collected, processed, and shipped widely for decorative purposes. Tanoak forests support many highly prized mushroom species including boletes, coral fungi, and a matsutake. Black oak is a key component of low to mid elevation forests on dry sites. It grows individually or in groves and often acts as a nurse tree to conifers. Acorns of both species are important wildlife food sources, and large trees are significant for cavity nester use. Both species have important ties to Native American cultures. Pacific madrone occurs commonly in southwest Oregon, from the drier aspects in coastal forest communities to a wide array of sites in the interior valleys. Madrone wood is dense and hard and is used in small quantities for lumber and flooring. It is a popular firewood species. The smooth reddish-orange bark, shiny green leaves, and berries contribute to its value as an ornamental species. Birds and small mammals feed on its berries. Myrtlewood is considered the most valued and best-publicized hardwood species in the western United States. The wood is beautifully grained; items made from myrtlewood are widely marketed. Native Americans used all parts of the tree for food and medicinal purposes. Myrtlewood provides food and cover for many wildlife species including birds, small mammals, and deer. It is also used in landscape plantings. Evergreen huckleberry is widely distributed from northern California through Oregon, Washington and into British Columbia. It is a community dominant throughout much of its range, is an important browse species for elk in the Coast Range and in southern Oregon, is dug for use in landscape plantings, and its branches are collected and shipped widely in floral arrangements. It is closely related to blueberries and cranberries as well as other huckleberry species and could provide the bridge for movement of the SOD Phytophthora to these agricultural commodities. Rhododendron species are also widely distributed in the forests of Oregon and the threat to the rhododendron horticultural industry is considered real.

The SOD Phytophthora seems to prefer cool, moist conditions but has the capability of surviving through dry periods. Thus Oregon's climate, particularly in the Southwest Oregon coastal zone would appear to be quite favorable to the disease.

Many possibilities exist for movement of plant material and soil from infested areas into and around Oregon. Oak firewood is routinely sold as a commercial product in stores and cut for individual homeowner use. Lumber, wood chips, and other wood products move readily through Southwest Oregon. Boughs and leaves of host plants are collected and sold worldwide. Mushroom gatherers move among forest stands. Hikers, mountain bikers, hunters, and other recreationists have access to acres and acres of forestlands. Nursery products are shipped widely.

KEEPING OREGON SOD-FREE

Quarantine
Preventing new introductions of SOD in Oregon is the best way we have of protecting our plants and forests. The Oregon Department of Agriculture has established a quarantine to protect Oregon's natural resources and agricultural industries from the artificial spread of Sudden Oak Death. All plant and plant parts of the known affected species as well as those plant species that are found to be susceptible in the future AND any associated soil are covered by this regulation. For example, tanoak, black oak, coast live oak, Shreve's oak, evergreen huckleberry or rhododendron nursery stock, logs, lumber, bark chips, firewood, sawdust, or mulch are prohibited from coming into Oregon from any of the infested California counties unless they have been kiln-dried, heat-treated or sterilized. Plant products from affected species that originate in counties adjacent to those infested also must be treated or inspected and certified as disease-free. Oak or other susceptible species products coming into Oregon from any other parts of California must have a certificate of origin supplied by the California Department of Food and Agriculture.

Oregon's quarantine will also affect the infested areas within Oregon. Movement of host material will be prohibited from infested properties. Logs, firewood, boughs, other special forest products and any other host materials will be affected. Additional areas outside actual property lines may be included in the quarantine area. Details of the boundaries of the quarantine within Oregon are still being worked out.

For those who are interested, the full text of the SOD quarantine is available by visiting the ODA website: www.oda.state.or.us/Plant/ppd/sod/EQ_SOD.htm.

Other quarantines have also been established against Sudden Oak Death. Information on restrictions can be viewed on the following websites:

California
www.cnr.berkeley.edu/comtf/pdf/monthlyreports/COMTF%20Update%20Jun_2001.pdf

Canada
www.cfia-acia.agr.ca/english/plaveg/protect/dir/d-01-01e.pdf

South Korea
www.cnr.berkeley.edu/comtf/press/nyti072501.html

Eradication
Eradication is the total removal of a disease from a site. The Oregon Department of Agriculture is proposing an eradication project to cut the symptomatic and susceptible plants in the four infested areas in Southwest Oregon, cut the susceptible plants in a buffer area surrounding each of the infested areas, and burn the plant material on site. Plant pathologists believe that eradication has a good chance of being successful at the early stages of a new introduction, particularly when infested sites are relatively small and discrete. Such appears to be the current case with Sudden Oak Death in Southwest Oregon. Details of the treatment need to be worked out and environmental analysis on federal lands will need to be completed before any eradication is attempted. The target for completion of the eradication project would be prior to fall/winter rains since it is believed that wet conditions facilitate movement of P. ramorum into other areas.

WHAT CAN YOU DO TO HELP?

DO NOT transport oak firewood or other potentially infected plant materials from diseased areas in California or Oregon into other areas in Oregon.

IF you visit diseased areas, wash your vehicle, mountain bike, shoes, etc. before entering disease-free areas.

On Private or State lands: REPORT host species with SOD-like symptoms that occur on private or state lands to the Oregon Department of Agriculture (503 986-4666), the Oregon Department of Forestry (503 945-7397) or your local county extension agent.

On Federal Lands: REPORT hosts with SOD-like symptoms on federal lands to the USDA Forest Service Southwest Oregon Forest Insect and Disease Service Center (541-858-6126).

Photos:

In cooperation with the Oregon Department of Agriculture, Oregon Department of Forestry, and Oregon State University, the USDA Forest Service, Southwest Oregon Forest Insect and Disease Service Center will continue the SOD surveys we began in 2000 and will monitor the known infested areas. Phytophthora ramorum, the cause of Sudden Oak Death represents an extremely severe potential threat to forest plants in the Pacific Northwest. Our profound hope is that SOD can be managed quickly and effectively in affected areas.

Ellen Michaels Goheen
Plant Pathologist
Southwest Oregon Forest Insect and Disease Service Center
541 858-6126
egoheen@fs.fed.us
8/20/01