News from the Sudden Oak Death Science Symposium (Monterey, US, 2002-12-15/18)
Last December, a symposium on Sudden Oak Death took place in Monterey, US, to present the state of art on this disease. Abstracts and posters presented are available on Internet and the EPPO Secretariat has tried to select new information among the vast amount of data presented:
Situation in USA
Sudden Oak Death occurs in 12 counties from Humbolt to Monterey, and is particularly severe in Marin, Santa Cruz and Monterey counties. It seems that the disease continues its progression, but it is not known whether the rate of spread is increasing or decreasing. For example, a study done in 2 plots in Marin county from 2000 to 2002, showed that the number of symptomatic trees and of dead trees has increased. In California, the Lauraceous plant Umbellularia californica (California bay laurel) is believed to play an important role in spreading the disease (as P. ramorum sporulates abundantly on it). In many sites, insects such as bark and ambrosia beetles are consistently associated with symptomatic trees. Studies showed that P. ramorum could be found in water streams crossing infected zones during rainy periods (winter/spring). It could also be retrieved from hiking trails and shoes during rainy periods but not during the dry summer period.
Sudden oak death was discovered in July 2001, near Brookings (Curry county), killing Lithocarpus densiflorus (Fagaceae). Die-back was also observed on wild Rhododendron pacificum and Vaccinium ovatum. In contrast to the situation in California, Umbellularia californica was rarely infected by P. ramorum. Eradication measures are being applied in Oregon (destruction of infected plants). Ongoing surveys have showed that the disease is limited to an area of 23 km² of forests, and that it does not occur in nurseries, plantations or botanical gardens. In 2002, it was found at 10 new sites, but these remained small and close to previous ones.
Concerns are expressed by other States in America which are free from the disease and measures are being taken to avoid introduction. It is felt that movement of nursery plants (Rhodendron, Vaccinium ovatum), and of Pseudotsuga menziesii (grown in Christmas tree farms) could represent risky pathways.
Situation in some European countries
Situation in the Netherlands
Since 1993, a Phytophthora has been found associated with twig blight in Rhododendron, and sporadically on Viburnum. In 2001, it was identified as P. ramorum. From 1993 to 2000, 18 samples of Rhododendron (15 locations in nurseries and public gardens) and 1 sample of Viburnum were found infected. Surveys continued in 2001 and 214 sites with Rhododendron and Viburnum were inspected. As a result, 11 positive cases were found in nurseries and garden centres (out of 78 inspected) and 7 in public or private gardens (out of 136). Inspections were also carried out on the 15 locations where P. ramorum had previously been found. On 5 of them, no more host plants were grown. Infections were found at one nursery (out of 2), and at 3 public and private gardens (out of 8). No symptoms were ever seen on nearby Quercus, Fagus or Castanea trees. In the Netherlands, P. ramorum was only found on Rhododendron (particularly R. ponticum and R x catawbiense) and on V. x bodnantense. No spread to other nursery plants was observed. Eradication measures were taken in nurseries: all infected plants (and neighbours within a radius of 1 m) were destroyed, remaining plants were treated and re-inspected within the following 3 months, where infection was found it was then prohibited to grow host plants for 3 years. Control measures were applied in public and private gardens: infected plant parts were pruned and destroyed, fallen leaves removed, and wound treatments were applied. In 2002/2003, inspections will be carried out on all 700 nurseries growing Rhododendron/Viburnum with special attention given to nearby Quercus, and to 2000 locations on Rhododendron in public or private gardens.
First findings in France
In April 2002, symptoms caused by P. ramorum were observed in several garden centres on Rhododendron. Affected plants showed brown spots on the leaves, bud and twig necrosis. In 2002, a national survey was carried out in nurseries. About 300 samples were tested (isolation on selective medium, morphological characterization). P. ramorum was detected in 68 samples (63 from Rhododendron and 5 from Viburnum) originating from various locations in France. This is the first report of P. ramorum in France.
Since November 2002, emergency measures have been taken by EU Member States to avoid introduction and spread of P. ramorum (see EPPO RS 2003/040).
Comparison of European and American populations of P. ramorum
The phytosanitary situations observed in Europe and USA are clearly very different. In California and Oregon, tree death is observed on North American oak species and symptoms are found on several understory plants. More information is needed on the situation in nurseries. In Europe, the disease has never been seen on oak trees or other Fagaceae, but P. ramorum is isolated from a few nursery plants (Rhododendron, Viburnum, and more recently on a few other species, see below). Many comparative studies are being conducted to try to understand why the situations are so different. Genetic studies have shown that USA and European populations belong to the same species P. ramorum. But, they are of different mating types (A2 in USA, A1 in Europe) and all attempts to cross European and American isolates have been unsuccessful. In addition, each population appears genetically rather homogeneous. Therefore, it is hypothesized that the pathogen was separately introduced into these two regions from a third area which remains unknown. Preliminary studies have also been initiated to compare the pathogenicity of European and American populations, but it is premature to give any conclusion.
8 plant species have been reported as new host plants of P. ramorum (most of them in USA).
Quercus chrysolepis (canyon live oak) - Fagaceae
Toxicodendron diversilobatum (poison oak) - Anacardiaceae
Rubus spectabilis (salmonberry) - Rosaceae
Rhamnus purshiana (cascara) - Rhamnaceae
Corylus cornuta (California hazelnut) - Corylaceae
Pittosporum undulatum (Victorian box) - Pittosporaceae
Pieris (found in UK, since this Symposium, P. ramorum has been found in UK on other ornamental shrubs see EPPO RS 2003/039) - Ericaceae
Trientalis latifolia (Western starflower) – Primulaceae, first herbaceous species confirmed as a host
Inoculation tests in the laboratory showed that a very wide range of nursery plants, European and Mediterranean shrubs or tree species are susceptible to P. ramorum, but this data is difficult to extrapolate in field conditions.
Several papers and posters were presented on assays to detect P. ramorum specifically in plant tissues (PCR tests, AFLP, isozyme profiles, gene sequences etc.).
Chemical treatments against P. ramorum are being studied (e.g. tree injections, control of insects associated with sudden oak death), but they will probably be difficult to apply in natural environments. Studies were also done on washing techniques for vehicles to limit the spread of the disease in infected areas, and also on the efficacy of composting for the disposal of infected green waste.
Scientific discussions will soon continue on Internet, as an on-line Conference ‘Sudden oak death – how concerned should you be ?’ will take place from the 21st of April to the 4th of May 2003: http://sod.apsnet.org/default.htm
Abstracts of papers and posters presented at the Sudden Oak Death Science Symposium – The state of our knowledge (Monterey, US, 2002-12-15/18). http://danr.ucop.edu/ihrmp/sodsymposium.html
McCreary, D. (2002) Symposium Summary. http://danr.ucop.edu/ihrmp/sodsymp/summary.html