Sudden Oak Death

Class: Oomycetes
Order: Peronosporales
Family: Pythiaceae
Species: Phytophthora ramorum

This month’s article is a little different because it’s about a pest problem that we do not have yet. However, sudden oak death is often in the news and is now a concern for Washington State. If you read this article regularly, you recognize that education and information is the most useful pest-managing tool. So it doesn’t hurt to learn about it now before it comes here.

History
The name “sudden oak death” sure sounds horrible. If I picked up the newspaper and read the headlines about a new disease “Sudden Todd Death,” I would probably never step outside of my house again for fear of my life. Sudden Oak Death (or SOD) was named so because in 1995 epidemic proportions of tanoaks died suddenly in Marin County, California. Lush green tanoak canopies suddenly turned crispy brown. Large seeping, bleeding cankers were found on the trunks of the trees. Disease was suspected but no critter could be identified. It wasn’t until 2000 the suspect disease was identified as a Phytophthora but the species was un-described. Here in the Pacific Northwest, we recognize other Phytophthora species as “root rots.”

When this problem was emerging in California, plant pathologists in Germany and The Netherlands were describing a new species of Phytophthora that was found infecting ornamental rhododendrons in Europe; the scientists named it Phytophthora ramorum, meaning the ‘destructor of branches’. In 2001, it was realized that this newly described European species was the same as the Phytophthora found in California. Once the disease was diagnosed and named, research into the extent of its range, potential hosts and regulation began. We are still in the early stages of understanding the disease.

Currently, SOD is living in twelve counties in California and one county in Oregon. In Europe, P. ramorum has been found in Germany and The Netherlands. Interestingly, the California strain of P. ramorum is a different mating type than the European strain. A mating type in Fungi is somewhat analogous to our different sexes; only different mating types can join and reproduce sexually.

Before the spring of 2003, SOD was perceived as a forestry problem. It has now been found in nurseries in Portland, OR; King County, WA; and New Westminster, B.C. To make things more confusing, both the European and Californian mating types were found in plant materials at some of these nursery locations. Because it has a broad host range, and is moving around via nurseries, P. ramorum can impact many plant species important to plant nurseries and Whatcom County gardeners, such as rhododendrons, madrone trees, Douglas-fir trees, camellias, and vaccinium plants.

Biology
During a recent conference about SOD at a WSU Puyallup workshop, a gentleman stood up and yelled, “My God, what you scientists are describing to us sounds like a fungus gone insane!” This comment was prompted after the gentleman learned the biology of P. ramorum.

P. ramorum is a hearty pathogen. It has many strategies for getting around, infecting new plants, growing fast, reproducing, and weathering bad conditions. The life cycle of P. ramorum is complex depending on the host, environment and weather conditions. The following website had a diagram illustrating the potential life cycle of P. ramorum (http://cemarin.ucdavis.edu/ecology.html).

Basically, P. ramorum can infect and germinate by three structures, through zoospores, sporangia and chlamydospores. Sporangia can disperse and infect plant tissue by direct contact or being blown through the air. Zoospores are short-lived but very mobile. The zoospores have flagella (tails) that they use to swim through water. Splashing rainwater or blowing air also moves zoospores around. They can form cysts and stay viable in soil and infect new plant tissue. Chlamydospores are the dormant stage of this fungus. Chlamydospores can lie dormant for some time and can tolerate drought. This dormancy strategy is why it is difficult to grow plants in previously infested soils; the fungus can persist for some time without any host.

Because P. ramorum has diverse ways of reproducing, containing it may prove difficult. P. ramorum can be spread to other hosts through air, water, rain, soil and plant debris. People can move it via plants, plant material, soil, plant products, wood, woodchips, dirty shoes, and water. P. ramorum does best in cool, wet climates (like ours).

This disease is fast. Sporulation can happen quite rapidly after inoculation. So once a plant is infected, P. ramorum can already produce dispersal stages to infect other plants. Cankers have girdled tanoak trees within four months of inoculation.

Symptoms
Diagnosing SOD is very difficult. The symptoms can resemble many other plant diseases or problems. Plant pathologists have to use genetic-characterizing methods to accurately identify P. ramorum. The symptoms of SOD are variable among hosts. Currently there are 38 species of plants that are susceptible to P. ramorum, 22 of which are quarantined by the federal government. For a complete host list, visit: http://www.suddenoakdeath.org/. Visit this regularly, it is quite amazing how diverse the hosts are and I expect that this list will grow.

Sudden Oak Death doesn’t cause death in all plant species that are affected. In fact, the California oaks, like tanoaks, are the hardest impacted. P. ramorum can infect limbs, trunks and leaves, depending on the host. Infection of woody tissue is seen through the formation of cankers. The bark will produce oozing sap as the living tissue underneath the bark dies. In oaks, the collapse of the tissue can happen rapidly, resulting in the sudden decline of the tree’s canopy turning from green to brown. This type of damage can cause dieback in the canopy and possible girdling of the tree or shrub resulting in death.

P. ramorum can also infect leaves and stems. Leaf symptoms appear as brown spots or patches, particularly on the leaf tips where water collects. In some plant species, the leaves appear to be the only susceptible plant part. Leaf infection rarely results in the plant’s death but does allow the disease to build up a large amount of innoculum. Infection of new growth is also used as an indicator of P. ramorum. Infected new growth will appear wilted, as in the case of Douglas fir.

Washington, Oregon, California and British Columbia are working hard to restrict the movement of infected material. Through regulation and public awareness, the progression of SOD will slow. A large effort is being invested into this horticultural nightmare so that, by the time it reaches us in Whatcom County, we can treat this as any pest problem. Stay educated about sudden oak death by visiting: http://www.suddenoakdeath.org/.