Sudden Oak Death Science Symposium
Robert Long, OSAF Forest Science Coordinator

A symposium devoted to sudden oak death was held in Monterey, California in December, and attracted almost 300 participants. Additionally, a field tour enabled those of us from the eastern U.S. to see the disease under diverse conditions in coastal areas near Monterey. Considerable new information was provided at the meeting concerning pathogen genetics, epidemiology, and control. Much of this has implications for eastern oak forests.

Phytophthora ramorum is the causal agent of sudden oak death and has only been found in western North America and Europe. P. ramorum isolates in the U.S. are a different mating type from P. ramorum isolates found in Europe. The pathogenicity of these two types may also differ slightly. It is thought that small differences between the European (A1 mating type) and North American (A2 mating type) P. ramorum isolates may be due to a founder effect, where two narrow and slightly distinct lineages were introduced independently in Europe and California. To date, European P. ramorum isolates have only been recovered from nurseries on Rhodendron spp. and Viburnum spp. None have been recovered from non-cultivated or wild vegetation in Europe. Extensive surveys have not been conducted, but will be initiated this year. In California, P. ramorum has only been found in one nursery, but is widespread on natural vegetation in coastal areas. To date the sexual stage of P. ramorum has not been found in the U.S. In one study, 82% of the isolates from California and Oregon were a single genotype and indicate a largely clonal population in North America. Only asexual reproductive propagules, sporangia with zoospores and chlamydosprores, are found in the U.S.

The fungus has not been observed to sporulate on bole cankers that affect woody hosts like tanoak, Lithocarpus densiflorus and coast live oak, Quercus agrifolia. However, the fungus does sporulate on leaves of California bay laurel, Umbellularia californica, but only when the leaves remain attached and alive. P. ramorum spores were only very infrequently recovered from litter during the dry season. Madrone, Arbutus menziesii, another understory shrub, is highly susceptible to infection however it does not produce abundant spores that could facilitate spread of the pathogen. Identification of the pathogen can be difficult and various polymerease chain reaction (PCR) protocols that rely on DNA analyses are being used to detect the fungus along with culturing. Presently, the Animal Plant Health and Inspection Service (APHIS) is evaluating the PCR protocols in order to select one or more that will be used for detection monitoring. New detection surveys will be conducted this spring and summer in the eastern U.S.

The spread of P. ramorum in California seems to be highly dependent on accompanying infection of California bay laurel, Umbellularia californica. Without the presence of this shrub the pathogen does not appear to spread readily. The primary infection period is from January to April, but there is still some infection from April through July. There is very little infection during the summer months. Leaf assay tests indicate that it takes a large inoculum load to infect bay laurel, but that once infected the leaves will produce abundant spores. Eradication of the fungus in Curry County, Oregon, has been accomplished by cutting and burning affected areas. Additionally, while bay laurel occurs in this area, so far only one leaf was found infected with P. ramorum. While a few infected trees are still present in or near the treated areas in Curry County, there was optimism about the success of this eradication treatment.

There is some evidence that bark and ambrosia beetles may play a role in spreading the pathogen. Bark and ambrosia beetles (Coleoptera: Scolytidae) consistently colonize the bleeding areas of infected trees while the trees remain functional with green foliage. These insects usually attack recently killed trees and branches, but have not been reported to colonize living trees.

The work of Everett Hansen and colleagues at Oregon State University has shown that P. ramorum likely is associated with three different patterns of disease development: 1) lethal bole cankers on a few species of Fagaceae such as Q. agrifolia and Lithocarpus densiflorus, 2) P. ramorum leaf blight occurs when many angiosperm species are exposed to sufficient inoculum levels, and 3) P. ramorum shoot dieback results from foliar infection and/or direct infection on stems on a limited number of species such as tanoak, madrone, some Vaccinium and Rhodendron species, and several conifers.

There is a great deal we don’t know about the threat of P. ramorum in the East. While we know that seedlings of red oak, Q. rubra, and pin oak, Q. palustris, are highly susceptible in inoculation studies, we don’t know how this infection process would proceed under natural conditions. We don’t know if we have an eastern species like bay laurel on which the fungus will sporulate prolifically and which will serve to vector the pathogen across a wide range of conditions. Leaf bioassays (done by dipping leaves into a spore suspension) showed that mountain laurel, Kalmia latifolia, was susceptible; however, sourwood, Oxydendrum arboreum, was highly susceptible in these tests. None of these tests have measured spore production on these hosts. Because of its wide host range, other potential hosts need to be evaluated including eastern Vaccinium species, various Lonicera species, and native Viburnum species.

Clive Brasier, a world authority on Phytophthora fungi, gave an interesting presentation on regulatory control of exotic pathogens. His criticism is that the system has structural rigidity, is list rather than process driven, lacks flexibility, and generally responds to ripples after a splash instead of preventing the splash. Only 10-20% of possible pathways are covered by the existing systems. He suggested we needed a much more proactive approach, with consequences to those who are responsible for introducing a pest or pathogen (“polluter pays” concept). He likened the prevention of exotic pathogen introductions to the war on terrorism: 1) it is a never-ending task, 2) potential losses could be quite high, 3) always must be 2 steps ahead of the enemy, and 4) effective intelligence is the first line of defense.

Finally, Dr. Wayne Sinclair, a forest pathologist now retired from Cornell University, was present at the meeting. When asked whether he was concerned about the spread of this pathogen to the eastern U.S., Dr. Sinclair replied that he wasn’t sure yet whether sudden oak death was going to be like Dutch elm disease or whether it would be more like scleroderris canker. A considerable amount of additional research will be needed to determine the potential threat of this pathogen to eastern forests.

If you want to read the abstracts of the oral presentations and posters given at this meeting, you can view them at: