THE TREE DOCTOR (aka The Plant Doctor)

Sudden Oak Death (Phytophthora Ramorum) - A Disease We Don't Want

Sudden Oak Death - SOD

Some of you may have heard of Sudden Oak Death (SOD), caused by Phytophthora ramorum (abbreviated P. ramorum). SOD was discovered in the Netherlands and Germany in 1993 and then in central California in 1995. Since that time, the pathogen has spread dramatically through several California counties, killing 10,000’s of Tanoaks and coastal live Oaks. It has also been found in Oregon, Washington and Canada. In the March issue of Science, it was announced that the SOD might have been inadvertently shipped from a California Nursery (one of Monrovia’s sites) to approximately forty states. Michigan was apparently one of those states that could have received infected nursery stock. Thus far, Florida and North Carolina have confirmed the presence of P. ramorum in their respective states and have taken aggressive action to eliminate all potentially infected plant material from California. Florida has advanced a more aggressive stance of instituting a quarantine against all plant material from California. 

What is Phytophthora?

Phytophthora is a fungus or fungus-like microbe that is fairly common as various species. For example, the species, Phytophthora infestans causes late blight on potato and was primarily responsible for the Irish potato famine in the mid 1800’s, when approximately one million Irish people died of starvation and approximately one million people emigrated to other countries. Many Irish emigrated to the North Americas. Michigan farmers still spray pesticides to control late blight on potato crops to this day. P. cinnamomi is a common forest root rooting organism in the US. There are many other native or common phytophoras which cause plant disease around the world. 

The origin of P. ramorum is not understood, but it behaves as though it’s a foreign invader. In general, phytophoras produce a variety of spores and a fungal-like body, called hypha or mycelium (essentially microscopic tubes or roots). All Phytophthora spores and bodies are microscopic and cannot be seen without the aid of a magnifying device such as a microscope. Obviously, Phytophthora cannot be easily detected and is probably on the order of at least a 1000X smaller than an insect such as the Emerald Ash Borer. Phytophthora also has  a much greater potential for reproduction than an insect such as the EAB, an adult female EAB may produce 60-80 eggs in a season. Each Phytophthora propagule is capable of producing 1000’s of spores in a short time and this can be repeated many times in a season. Phytophthora may be carried by the wind as spores or in dust/soil particles or plant debris. It can be transported in water or tracked on shoes or equipment. And, obviously, it can be transported with plants, even without any signs of infection or evidence of its presence.

Host Range of Phytophthora ramorum:

Compared to such exotics as the Emerald Ash Borer and the Dutch Elm Disease fungus which have very narrow host ranges (ash and elm respectively), Phytophthora ramorum has a very broad host range. Generally, P. ramorum causes cankers on some host plants and foliar/twig blights on others. 

Some of the cankered hosts include: Tanoak (Lithocarpus densiflora), various Oaks, Coastal Redwood and Douglas Fir. Nothern Red Oak and Pin Oak have been tested and are quite susceptible, indicating the possibility that P. ramorum could easily become established in eastern and Midwestern forests and landscapes.

Some of the foliar blight hosts include Vaccinium sp., Rhododendron, bay laurel (Oregon myrtle), bigleaf maple, California buckeye, coffeberry, honeysuckle, camellia, etc. 

The canker host plants and blight host plants are currently fall under regulation by the USDA APHIS (Animal & Plant Health Inspection Service). Please bear in mind that the list of susceptible host plants and carriers continues to expand.

Symptoms Caused by Phytopthora ramorum:

On canker host plants such as Oak, cankers develop on the lower trunks of infected trees and may bleed a dark ooze. Secondary insects (ex. Borers) often invade cankered areas as do other fungal diseases, further complicating an accurate diagnosis of SOD. On canker hosts, symptoms result in brown foliage and hence plant death, usually taking about 1-2 years. Hence, the disease name, Sudden Oak Death, may be a misnomer. On blight hosts such as rhodendron, spots and blotches develop on the foliage. It may be very difficult to distinguish P. ramorum incited symptoms from those caused by other common, native disease organisms.

Immediate needs


1) SOD Task Force

A Michigan Sudden Oak Death Task Force needs to be formed immediately. This task force should be comprised of University Scientists, Government regulatory Officials, Industry Leaders, etc. The politics of such a task force should be kept to an absolute minimum by proper administration and a common goal to effectively deal with this serious threat.

2) Rapid, Accurate & Free Diagnostic Detection Service

Suspect cases of Sudden Oak Death (S.O.D.) need to be confirmed by a diagnostic system so that the whereabouts of P. ramorum can be determined for possible management and or eradication.
The service needs to be free so that sample submission is not hindered. Perhaps for encouragement, rewards
could even be given to those submissions leading to a successful detection of P. ramorum. Because of its characteristics, P. ramorum is far more difficult to detect than the Emerald Ash Borer.

3) Education of Professionals in Nursery and Landscape and the Public

MSU Extension should take a lead role in education of Plant Propagators, Plant Health Care Professionals and the public. Other entities should
also become involved in this endeavor to maximize our understanding and detection of P. ramorum before it
becomes established and/or widespread.

4) Research on Prevention, Management, Containment, and possible Eradication

Although eradication of a microorganism will be far more difficult than eradication of an insect such as the EAB,
there is much information that needs to be determined for managing P. ramorum in nurseries, our forests and landscapes.

5) Regulatory Measures: SOD is the SARS of the Plant World

Strict measures need to be implemented to ensure that SOD does not become established in Michigan, or if it is already established (which I suspect is the case), to contain its spread as much as possible.


Because Phytophthora is microscopic and hence is difficult to detect, because it can be transmitted very readily by a variety of means, because of its broad host range and because of its lethal nature for many of our valuable plants, Sudden Oak Death (S.O.d.) has the potential to become far more problematic than either the Emerald Ash Borer or Dutch Elm Disease. We need to address this serious threat to our environment now.

Contact Local Extension Office for Information About SOD:

MSU Extension Macomb Office (586) 469-5180
MSU Extension Washtenaw Office  (734) 997-1678
MSU Extension Berrien Office (269) 944-4126

Other Links


 California Oak Mortality Task Force  

Sudden Oak Death Diagnostics Guide with photos

Genome News Network: 8/19/04  Genome Could Aid Fight against Tree Disease

JGI:  Phytophthora ramorum

NewScientist.com 8/10/04 Hikers help spread sudden oak death

APHIS.USDA on Sudden Oak Death (PDF)

USDA Pest Alert: Sudden Oak Death