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Washington State University
WSU Puyallup Ornamental Plant Pathology

Plant Disease Research Programs

Plant Disease Research Programs

The Ornamental Plant Pathology program provides research and extension services for many plant diseases, with special emphasis on diseases caused by Phytophthora species.

Click on the image to be redirected to the program.

Sudden Oak Death
& Ramorum Blight

Symptomatic potted plants

Pacific Madrone

Pacific Madrone

Dutch Elm Disease
National Elm Trial

Puyallup National Elm Trial

Christmas Trees

Christmas Tree Research Program


The Pacific Northwest produces close to one third of the Christmas trees sold each year nationally.

The goal of our research program is to provide growers and retailers with research-based information that creates a high-quality Christmas tree products for consumers.

We strive to accomplish this goal by conducting applied research focused on reducing the impacts of pests and pathogens and increasing the longevity of trees after they are harvested.

Grower Resources

PNW Handbook Resources

WSU Extension Documents

Click on the images below to visit the WSU Extension publication website. These resources are available to download for free.

Recent Research Projects

Douglas-fir Twig Weevil

Christmas tree damaged by twig weevil

The Douglas-fir twig weevil recently emerged as a important pest of true firs in the Pacific Northwest. Outbreaks of this pest have impacted the tree export market by causing load rejections in markets with neighbor countries such as Mexico. The damage from these twig weevils also affect the quality of trees for the domestic market.

Our program, in collaboration with Oregon State University Extension, aims to:

  1. Reveal the diversity of twig weevils associated with different Christmas tree species.
  2. Increase our understanding of their life cycles to identify critical stages for control.
  3. Develop a guide for growers to identify and control twig weevil populations.

Feel free to contact us for more information.

Do you suspect twig weevil on your trees?

Click here (PDF) for more information on symptoms and how you can request we sample for this pest.


Contact Us

Heat Treatments for Imported Seed

Turkish and Nordmann fir seedlings are often grown from imported seed because of limited domestic sources. However, the importation of these seeds is often restricted and limited because of contamination by Megastigmus larvae.  For example, a substantial amount of seed is destroyed each year following the detection of these larvae because there are no currently approved treatments.

Therefore, a priority research topic for our program is to evaluate the potential of heat treatments to eradicate Megastigmus larvae in infested Turkish and Nordmann fir seeds. This research is made possible through support from the Christmas Tree Promotion Board and USDA APHIS.

<i>Megastigmus</i> larvae emerging from tree seed

Ongoing Research

Much of our pest and disease research has focused on the following topics:

  • the development and management of Annosus root rot,
  • the development of effective methods to reduce insect populations that restrict export markets,
  • and the identification of sources of trees with resistance to common diseases and insect pests such as:
    • Phytophthora root rot,
    • current season needle necrosis,
    • Grovesiellia canker,
    • spider mites,
    • and twig aphids.

Our postharvest research has focused on the identification of sources of true firs that have superior needle retention by testing detached branches.

Publications and Reports

  1. A Survey of Phytophthora spp. Associated with Abies in U.S. Christmas Tree Farms. K. M. McKeever and G. A. Chastagner, Plant Disease 2016 100:6, 1161-1169
  2. Postharvest Quality of Noble and Nordmann Fir Christmas Trees. G. A. Chastagner and K. L. Riley, American Society for Horticultural Science, 2003: 38:3.
  3. The Christmas Tree: Traditions, Production, and Diseases. G. A. Chastagner and M. D. Benson, Plant Health Progress 2000 1:1
  4. Christmas Tree Diseases, Insects, and Disorders In The Pacific Northwest: Identification and Management.1997. WSU Extension Publication MISC0186.

SOD Public Inquiry Guide

Tips to answer calls or questions from arborists, landscape professionals, homeowners, or the general public.

How to check for symptoms and the need to sample for Phytophthora ramorum

On trees:

Bleeding symptoms only occur on coast live oak, California black oak, Shreve’s oak, and tanoak. These species are not native to Washington State and are rarely seen here as ornamentals. Bleeding on maple, Douglas-fir, or other foliar hosts (with the exception of poison oak and Pacific yew) usually indicates a different causal agent. Leaf and twig symptoms do not occur on hosts in the red oak group, but may on tanoak and canyon live oak. Garry oak (white oak group) is not considered to be a host and only develops minor symptoms.

There are many diseases and pests that produce symptoms similar to those of Sudden Oak Death. For a full list with descriptions, see the Pest Identification document. Compare symptoms with those in the diagnostic guide or consult with a landscape professional for diagnosis.

On shrubs and other plants:

Phytophthora ramorum produces leaf blotches, twig cankers, and shoot die-back on many plants. In some cases the shoot die-back may be severe enough to kill the plants (e.g., huckleberry and madrone), but in general they do not usually die as a result of P. ramorum infection.

Foliar hosts may be damaged by fungi, other Phytophthora species, insects, the sun, or other environmental conditions, and these can cause symptoms similar to those of P. ramorum. This can make it very difficult to identify the pathogen on these species.

How to determine whether to submit a sample for testing:

Though you will not be able to identify SOD in the field, you can screen samples and decide if they should be sent in for further testing. When a client suspects Sudden Oak Death, use the key in this binder as a guide to determine if the sample should be submitted. There are also examples of what P. ramorum symptoms look like on a few common native and ornamental host plants in Washington State.

If you determine a sample should be submitted for testing the sample should be sent to WSU Puyallup for free diagnosis (postage not included). Please follow the approved procedure found at

What is the status of SOD in Washington State?

In June 2003, Phytophthora ramorum, the causal agent of sudden oak death was confirmed for the first time in Washington.

Since that time the Washington State Dept. of Agriculture (WSDA) has tested over 100,000 plants in hundreds of nurseries as part of the national SOD survey and trace forward surveys from production nurseries found to be infected with the pathogen.  As of March 31, 2014, APHIS no longer imposes P. ramorum regulatory requirements for the interstate movement of host nursery stock from certain nurseries located in the regulated areas of California, Oregon, and Washington. Nurseries that have not had P. ramorum detected in annual surveys since March 31, 2011 will not be required to be inspected and certified in order to ship regulated and associated host plants interstate. Also, nurseries located in regulated areas that do not contain, and that do not ship nursery stock listed as proven host taxa or as associated plant taxa are no longer required to comply with 7CFR 301.92. APHIS will, however, continue to regulate all interstate shipping nurseries located in quarantine areas of California and Oregon, including those that contain only non-host nursery stock.


Visit the WSDA SOD website for more information –

New Pr hosts found

Five new Phytophthora ramorum hosts were detected during a rare plant survey in February on Marin County Municipal Water District property when unusual Arctostaphylos symptoms were identified. Samples submitted to the California Department of Food and Agriculture (CDFA) Plant Pest Diagnostics Lab tested negative for root pathogens, but the leaves and branches tested positive for P. ramorum. Follow-up official samples were collected for testing by CDFA scientists, which resulted in the identification of five new P. ramorum hosts: Arctostaphylos virgata, Arctostaphylos glandulosa, chinquapin (Chrysolepsis chrysophylla), blackberry (Rubus ursinus), and chaparral pea (Pickeringia montana). Arctostaphylos virgata and Arctostaphylos glandulosa symptoms include leaf spots and necrosis, petiole dieback, stem lesions and cankers, and tip dieback. Many plants of both species were showing symptoms. Chinquapin (Chrysolepsis chrysophylla) symptoms include canopy dieback, leaf spots, and vascular discoloration. Many chinquapin were showing symptoms; two trees were confirmed positive. Blackberry (Rubus ursinus) symptoms include leaf spots and stem lesions. Chaparral pea (Pickeringia montana) symptoms include leaf spots, stem lesions, and thorn dieback. CDFA scientists are in the process of obtaining healthy container plants of each host species to confirm pathogenicity. A follow-up site visit to the water district is planned for July to observe disease progression on these hosts.

Vaccinium parvifolium (red huckleberry) was found P. ramorum positive for the first time from two samples taken at a Lewis County, WA interstate shipping nursery during their 2015 spring Federal P. ramorum Certification Program survey. The Lewis County facility was also positive in 2014 and had undergone a CCPA as well as extensive mitigation activities. A new CCPA has been conducted to determine possible ways the pathogen has continued to infect plant material; additional mitigation efforts will completed in 2015. As a result of the new host confirmation, Vaccinium parvifolium will be added to the federally recognized P. ramorum host and associated host list.

For more information on this and other news items, read the COMTF monthly newsletter.