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Selected highlights of the Sudden Oak Death Second Science Symposium
January 18-21 2005, Monterey, California, USA In California in 1995 the highly damaging tree disease Sudden Oak Death (SOD) was first noticed and in 2002 the causal agent was shown to be Phytophthora ramorum. P. ramorum is now found to be causing significant damage to trees and a range of native and ornamental plants in California, Oregon and a number of European countries, including the UK. Thanks to funding from BSPP and Forest Research UK I was able to attend the SOD second science symposium in January this year, and present a paper on the distribution and etiology of aerial stem infections of P. ramorum and P. kernoviae (another newly identified Phytophthora species) in the UK. The aim of the meeting was to provide an update on the current status of research being carried out on P. ramorum in the USA and Europe.
Approximately 300 people were in attendance from 11 countries. The initial sessions concentrated regulatory issues with particular reference to the nursery trade whilst the remaining sessions focussed on the biology (including fungal diversity, etiology, epidemiology and ecology), genetics (particularly diagnostics) and disease management, including eradication. Some of the key areas are covered below.
Biology One issue that arose in a number of the talks was the variation among and between the European and American populations of P. ramorum and the potential for sexual recombination. Several studies show European isolates to be predominately of A1 and American isolates of A2 sexual compatibility (or mating) type. In terms of phenotypic characteristics, the European and the American groups can be discriminated, European isolates having faster growth rates on agar over a narrow range and also producing larger mean lesion areas (i. e. having greater aggressiveness) when wound inoculations on tree stems are carried out. In addition, the colony morphologies of the European isolates tend to be uniform, whilst the American isolates are morphologically more variable and unstable. In contrast, other studies using microsatellites and AFLP analysis showed less neutral DNA variation in the Californian/Oregon population than in the European population. Several laboratories have successfully mated isolates of the European A1 and American A2 population types.
However, gametangial production is very sparse, and high abortion rates of oospores (60-70%) have been found. To date there has only been one finding of an A2 sexual compatibility type in Europe. This isolate displayed typical European phenotypic characteristics. In America, there have been reports of isolates of the A1 mating type and European phenotype in several nurseries in Oregon, and in one nursery in Washington, alongside American A2 types. In addition an isolate has been found that contains unique microsatellite alleles as well as alleles exclusive to both the European and American population types. This is believed to be a new and unique genotype resulting from a further introduction into America.
Evidence that P. ramorum can occur in the xylem of trees was reported in several papers, a finding which has implications for control and risk analysis.
One of the studies used magnetic resonance imaging and found that lesion development could extend up to 5cm into the xylem of Quercus agrifolia (coast live oak). In addition, the ray cells were shown to connect laterally between the inner xylem and outer periderm, and appeared to have P. ramorum hyphae growing through them. These ray cells also exhibited high water concentrations, which was also a feature of the damaged tissues of the lesion area. Interestingly, as a result of the P. ramorum surveys being carried out in the UK, California and Oregon, a large number of other Phytophthora species are being isolated from streams, soil foliar and bark samples. Although some of these species are considered to be endemic to the areas involved (e.g. P. citricola, P. cambivora and P. gonapodyides), they are “acting differently”, causing aerial stem lesions opposed to collar or root or shoot lesions.
In addition, previously undescribed species are being found which appear to be additional recent invasive causing significant stem lesions and tree mortality such as P. kernoviae sp. nov. in the UK, and endemics causing lower levels of damage, such as P. nemorosa sp. nov. in the US.
Diagnostics A lot of effort is being put into the development of DNA-based diagnostic systems including the field based TaqManÂ® real time PCR methods, single strand conformational polymorphism (SSCP) and microsatellites. Although all the methods discussed have potential for use in diagnostics, most also have their own particular limitations.
Completion of the genome sequence of P. ramorum was reported. Its possible implications for use was discussed in general terms. One such application was the possible identification of improved genetic markers, such as noval microsatellite markers for diagnostics.
However in Oregon, where the disease is confined to a relatively small area in the south-west (Brookings), an intensive eradication programme was established after the initial discovery of P. ramorum in July 2001. Initially the approach used was to fell and burn every potential host within 20m of a diseased plant, with all the litter being raked and burnt. Post-eradication surveys found that P. ramorum had survived on sprouts associated with the cut stumps of tan oak (Lithocarpus densiflorus) and other hosts. These are now treated with 10% glyphosate, a herbicide, at the time of felling.
Although the number of disease sites has increased since the eradication programme began, with 7 sites still undergoing treatment, the number of infected trees is decreasing. However, even where the eradication of P. ramorum on plant hosts appears to have been successful, the fungus is still detectable in the soil and in the streams.
In the UK the vast majority of P. ramorum infections have been found in nurseries and retail premises (c. 320). Around 75% of these have now been eradicated. However, outside nurseries, on managed and unmanaged land, only 11 of the 60 outbreaks have been eradicated to date.
After the meeting a number of delegates took part in a series of informal field trips to the Big Sur outbreak area to the south of Monterey. I also had the opportunity to visit ecologically distinct outbreak areas in central and Northern California and the eradication sites in south-west Oregon. In addition a visit was made to Oregon State University, Corvallis to see research in progress on SOD and to US Forest Service, Medford, Oregon to see control of Phytophthora lateralis on Port-Orford Cedar (Chamaecyparis lawsoniana).
Dr Anna Brown Forest Research, Alice Holt Lodge