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Third Sudden Oak Death Science Symposium, Santa Rosa, California, 5-10 March 2007
Over 270 delegates from around the world gathered in Santa Rosa in early March 2007 for the Third US Forest Service-promoted Science Symposium on ‘Sudden Oak Death’ caused by the previously unknown invasive forest pathogen Phytophthora ramorum. The first Symposium was held in Monterey in December 2002, two years after the initial discovery by researchers at UC Davis and UC Berkeley that P. ramorum was the cause of the disease. The second Symposium, also in Monterey, was held in January 2005.
P. ramorum is believed to have been introduced into western North America and Europe on imported nursery stock, probably around 15-20 years ago. Its geographic origin remains unknown. A feature of the pathogen is that it attacks a very wide range of shrubs, trees and ornamentals. In the nursery trade, susceptible species such as rhododendrons and camellias act as vectors for wider dispersal. In California, Oregon and some other parts of the Pacific north west, it is spreading both within the nursery trade and across large areas of coastal forest. In Europe it is mainly confined to nurseries but has also reached the wild in some countries, notably in Cornwall UK where along with another new invasive P. kernoviae it is spreading from rhododendrons onto beech and other broadleaved trees.
In Californian forests major foliar hosts for P. ramorum, on which sporulating lesions develop, are the tree species California bay (Umbellularia), madrone (Arbutus) and tanoak (Lithocarpus densiflora). Major tree stem hosts, on which bleeding lesions develop (but do not sporulate), are coast live oak (Quercus agrifolia) and tanoak. To date, over a million tanoak and coast live oak trees have died as a result of P. ramorum stem infections. The disease develops in different types of coastal forest ecosystem. The first afternoon of the meeting comprised a half-day field trip to see Sudden Oak Death in a tanoak/coast redwood system.
Presentations and discussions centred around public safety and fire control problems resulting from heavy tanoak mortality; the impact of tanoak mortality on native American customs; the impact of acorn loss on wildlife ecology; and experiments on chemical prophylaxis in tanoak using phosphites.
In reality P. ramorum, is a symptom – and is also becoming something of a symbol – of a much wider issue – the increasing threat to the world’s forests and natural ecosystems from invasive pathogens. This threat is believed to be driven by two main factors: the growing international trade in plants; and loopholes in international biosecurity protocols. One major loophole in biosecurity is the way unknown pathogens, such as P. ramorum, are not covered by international quarantine schedules. The response to the Sudden Oak Death issue, therefore, has become something of a blueprint for our ability to limit the impact of a damaging invasive within complex forest ecosystems. It is also a test of our resolve to improve the effectiveness of quarantine regulations and procedures governing international plant movement. In consequence the problem is of interest to a very wide range of individuals and disciplines.
These include forest pathologists, landscape scale modellers, wildlife ecologists, molecular diagnosticians, native American tribes, fire control professionals, plant health regulators and nursery owners. Owing to the wide range of specialism among the delegates and the fact that there were no ‘parallel sessions’ at the Symposium, the meeting produced a stimulating, refreshing and sometimes challenging mixture of discussion and opinion.
On the day prior to the main indoor meeting a special session was devoted to the problem of phytosanitary questions relating to P. ramorum and conifers, the primary issue being the risk of spreading P. ramorum on exported conifer lumber. Formal presentations from seven panel speakers (Brenda Callan, Shane Sela and Eric Allen, Canadian Forest Service; Gary Chastagner, Washington State University; Everett Hansen, Oregon State University; Joan Webber, Forest Research, UK; and Matteo Garbelotto, UC, Berkeley) was followed by a lively and informative discussion. Information compiled will be used by the NAPPO Forestry Panel to review and draft a report on the impact of P. ramorum in relation to a trade in conifer products.
In the oral sessions of the main body of the meeting a total of 68 papers were presented. Eighteen papers were presented by European researchers working on P. ramorum or on other invasive Phytophthora species such as P. kernoviae, twelve of these by UK speakers. Eighty posters were presented in the poster session, including 5 from the UK. The Symposium was divided into 10 sessions: Wildland Update; Nursery Update; Regulatory Update; Nursery Research and Management; Landscape Monitoring and Mapping; Diagnostics; Biology and Ecology; Genetics; Modelling; and Landscape Management.
With such a large number and diversity of presentations, this report will only highlight the contributions from the UK and Ireland.
In the Wildland Update session, Joan Webber (Principal pathologist, Forest Research) presented an overview of the status of P. ramorum and P. kernovaie in Europe, covering disease status in the EC and non EC countries infected and the extent of spread of the pathogen onto trees and shrubs outside nurseries. The talk emphasized that rhododendron is presently the major host for the spread of these pathogens from foliage onto tree stems in the Northern Europe. Joan also highlighted a range of ‘new’ discoveries emerging from the EC Framework 6 RAPRA research project (http:// rapra. csl. gov. uk).
In the Nursery Update session David Slawson (Chief Plant Health Inspector, PHSI, UK) stressed that eradication and intensive monitoring has lead to a significant reduction in the numbers of infested nurseries in the UK in the past 3 years. However, incomplete application of EC emergency measures relating to P. ramorum by some EC member states was a continuing concern. In the following Regulatory Update, Stephen Hunter (Head of Plant Health, Defra) explained that the UK would be going through a process of review before going to ministers with recommendations on the future direction of policy for P. ramorum and P. kernovaie. The EC will be going through a similar review. The outputs of the EC FP6 RAPRA project coordinated by Joan Webber were crucial to both reviews. In the session devoted to Forest Quarantine issues, Clive Brasier (Forest Research) stressed the frequent failures of the current international plant health system as a consequence of its reliance on lists of known pathogens. In Europe, many of the recently introduced tree pathogens (e.g. P. ramorum, P. kernovaie, P. alni) were unknown to science until discovered as damaging agents. He stressed the need for changes in regulation at the pathway level, especially in regard to plants for planting.
In the session of Landscape Monitoring and Mapping, Judith Turner, David Lockley and colleagues (CSL, ADAS UK) reported on monitoring inoculum of P. ramorum and P. kernovaie at ‘wild’ outbreak sites in Cornwall. Using leaf baits and PCR, 1000 spores of P. ramorum can be consistently detected in water. Production of airborne inoculum peaked during the winter months with both species, and there was evidence of dispersal of P. ramorum beyond 50m from a rhododendron infection source, but only occasionally. Ian Wright (National Trust, Tregwainton) reported that P. ramorum is having a significant impact on historic and national collections of plants such as magnolia and rhododendron in the UK, with consequent effects for tourism and plant conservation.
Several UK and Ireland speakers contributed to the Biology and Ecology session. Anna Brown and Clive Brasier (Forest Research) reported how Phytophthoras, including P. ramorum and P. kernovaie, regularly infect the sapwood of various broadleaved tree species as well as the phloem and cambium, with implications for pathogen isolation success, pathogen survival, and risk of spread of Phytophthoras on transported timber.
Clive Brasier and Anna Brown also reported studies on the ability of zoospores to infect via the intact bark of tree stems, with possible evidence of resistance mechanisms expressed at the intact bark surface as well as in the phloem. Sandra Denman (Forest Research) and colleagues presented findings of asymtomatic infection and sporulation on foliage and fruits of ornamental plants such as rhododendron by both P. kernovaie and P. ramorum, with clear implications for movement of these pathogens, undetected, via the plant trade.
Joan Webber presented data on the frequency of inoculum on feet of people exiting P. kernovaie or P. ramorum infested sites in Cornwall, arguing that human and animal vectors and machinery may be an important component of spread. She also provided evidence of persistence of P. ramorum and P. kernovaie inoculum in leaf litter coming from infected rhododendron bushes at woodland sites. Carmel O’Connor (Gallway-Mayo Institute of Technology, Ireland) reported detection of P. ramorum on rhododendron in Killarney National Park using water baiting techniques.
In a session devoted to modelling, Marco Pautasso and Mike Jeger (Imperial College London and Wye) compared the spread of P. ramorum in nurseries to complex networks. They reported that the process of P. ramorum spread on rhododendrons may represent a ‘scale-free’ network, which would show a lower epidemic threshold than other types of complex network i. e. epidemics would be more easily initiated. The final morning of the 5-day Symposium was dedicated to a ‘Research and Management Needs Assessment’, based on information collated from delegates beforehand. The delegates were divided into two groups dealing with Horticultural needs and Forestry needs. Again a series of lively discussions ensued. The information is still being reviewed and a record of the discussions will be circulated to delegates.
Selected Powerpoint presentations and other information from the Symposium will be available from the web site of the California Oak Mortality Taskforce: http://nature. berkeley. edu/comtf/. A Symposium Proceedings will be published in due course. The organisers of the Symposium, in particular Susan Frankel, Mark Stanley, Janice Alexander and Katie Palmieri of the US Forest Service (Pacific South West) and the California Oak Mortality Task Force, are to be congratulated on a highly successful and stimulating meeting.
Clive Brasier Forest Research