Sudden Oak Death Syndrome (SODS)

The creation of a thousand forests is in one acorn.
-  Ralph Waldo Emerson

Known by several names including Sudden Oak Death, ramorum blight, ramorum leaf blight and ramorum dieback, Phytophthora ramorum (“Phy-TOFF-thoruh”) is the causal agent of a lethal disease that threatens west coast woodland diversity and the international nursery industry.

A little biology:
Although it is generally referred to as a fungi, P. ramorum is in the kingdom Protista, belonging to the group called Oomycetes, which are commonly called water molds. These organisms are biologically different but have similar characteristics as fungi. Two main differences are; 1) oomycete cell walls are composed of cellulose as opposed to chitin in the fungi, and 2) oomycetes have a swimming spore stage; hence the moniker ‘water molds’. The pathogen thrives in cool, wet climates (sound familiar?). You can imagine the effect our Northwest coastal weather has on a soil borne pathogen capable of traveling with water.

A little history:
Globally we are actually experiencing our third historic battle with an oomycete plant parasite; the first one, Phytophthora infestans, causes late blight of potato, and was the cause of the Irish potato famine from 1845 to around 1860. As a result of this disease, over one million people immigrated to other countries, so if you are of Irish descent, your ancestors probably came to America due to this. In the late 1870’s, Plasmopara viticola, the causal agent of downy mildew in grapes was accidentally introduced to Europe from North America and almost wiped out the entire French wine industry. The saving grace was the serendipitous discovery of a mixture of lime and copper sulfate that a grape farmer applied to his grapes along the roadside to stop inquisitive taste testers from freely sampling his wares. This accidental find has the notoriety of being the first known fungicide and is still in use today as Bordeaux mixture.

Where Phytophthora ramorum has been found:
The pathogen first appeared in California in 1995 and has since caused the deaths of tens of thousands of oaks and tanoaks in numerous coastal counties of California. It has been found in south-coastal areas of Oregon totaling approximately 70 acres and has continued its spread. In Oregon, these areas have now been clear-cut and burned in an attempt to eradicate it.

In June 2003, the pathogen was found for the first time in Washington State and has now been detected in over 20 nurseries of western Washington but thankfully not in our State’s urban or natural forested environments. The United States Department of Agriculture (USDA) has issued Emergency Action Notifications to all infected nurseries ordering the nursery to follow a federal eradication protocol. There have also been perimeter inspections around the contaminated nurseries with no infected plants found. In Europe, P. ramorum has been identified on nursery plants in Germany, Spain, France, Poland, Belgium, Sweden, and Italy. In the UK and Netherlands, it has been found both in nurseries and on forest trees.

Host preference:
Wild western forests are not the only cause of concern, for P. ramorum has no discerning taste of hosts and is substantially impacting the nursery industry internationally. This pathogen has an incredibly wide host range, with at least 38 species in 12 different plant families confirmed as susceptible (see an exhaustive list at http://www.aphis.usda.gov/ppq/ispm/pramorum/pdf_files/usdaprlist.pdf”). This is a very aggressive pathogen and can infect and kill otherwise healthy trees. There are two types of hosts for the pathogen, foliar and bark canker hosts. It infects the foliage and branches of various familiar ornamental nursery plants, such as Pieris and Rhododendron species. Although these foliar hosts become infected they do not necessarily die from the disease, but play a key role in its survival and spread, by harbouring its innoculum for later dispersal. Bark canker hosts include oaks, and they become infected on the trunks. Tanoaks can be infected both ways (http://cemarin.ucdavis.edu/symptoms.html). Cankers are the most lethal and frequently lead to death, since cankered trees are often attacked by secondary invading pests that sense the tree is in trouble.

Symptoms and Diagnosis:
Symptoms will not necessarily be consistent and will vary not only between species but also on each individual plant. These symptoms can include needle and tip blight, shoot-tip dieback as well as cankers and leaf spots. However, it is easy to misdiagnose this problem, since many other plant problems will exhibit similar symptoms. Environmental abiotic factors such as drought, sunburn and high soil pH may produce the same symptoms as well as other biotic organisms such as insects or other types of pathogens. The only way to substantiate the presence of P. ramorum is through diagnostic laboratory testing http://www.puyallup.wsu.edu/plantclinic/samples/ppd.html.