SEARCH

SEARCH BY CITATION

Keywords:

  • epidemiology;
  • genetic diversity;
  • microsatellites;
  • oomycetes;
  • plant health;
  • quarantine

Abstract

  1. Top of page
  2. Abstract
  3. References

Invasive species are, by definition, unwelcome and pathogenic ones, especially so. Tracing the origins and spread of Phytophthora ramorum, the devastating ‘Sudden Oak Death’ pathogen, in the forests and nurseries of Oregon has revealed differences between forest and nursery pathogen populations that suggest discrete sources of primary inoculum initiate each type of outbreak. New information on the ecology and evolution of this pathogen is presented that helps gauge the effectiveness of quarantine and eradication programmes.

Irrespective of whether a pathogen's host is an animal or a plant, effective management of the resultant disease hinges on an understanding of the pathogen's origins and ecology. Where does it come from? Where and how is it surviving? What are the sources of primary inoculum? How is it spreading over time and space? How rapidly is it evolving and how does it behave in natural ecosystems? Such questions are examined in an article in this issue (Prospero et al. 2007) on Phytophthora ramorum, a destructive pathogen of oak (and many other plant species) responsible for an epidemic in parts of California that has killed over a million oak and tanoak trees (California Oak Mortality Task Force web site http://nature.berkeley.edu/comtf/). Prospero et al. (2007) have designed new, highly polymorphic, simple sequence repeat (SSR) markers and used them to monitor the pathogen in the more restricted disease outbreaks in forests and plant nurseries in the state of Oregon (USA).

Clear parallels can be drawn between P. ramorum and the spread of the infamous and equally aggressive pathogen, Phytophthora infestans, a single dominant clone of which was inadvertently transported from its Central or South American origin to US and European potato crops (e.g. Gómez-Alpizar et al. 2007). This was 1843–1847, 20 years before Louis Pasteur's studies that lead to the widespread acceptance of germ theory. The description of this as the cause of blight thus marked the birth of plant pathology (Large 1940). Phytophthora ramorum, by comparison, is a newcomer, first discovered on rhododendron in Germany in 1995 but not formally described (Werres et al. 2001) until scientists realized its destructive potential. While its spread in Californian forests has been swift and devastating, a different pattern has emerged in Europe. In England and Wales, for example, it has also spread in nurseries but has, to date had a more limited impact on trees (see http://www.defra.gov.uk/planth/pramorum.htm).

The rate of spread and destructive potential of P. ramorum on a landscape scale has prompted an intense phase of research that has advanced the understanding of its pathology, aetiology, epidemiology and distribution. Control measures and legislation have helped check its local and international movement and its genome has been sequenced (http://www.jgi.doe.gov/). The paper in this issue marks a significant advance on two fronts. First, much needed, highly polymorphic, SSR markers are presented and second, their resolution is demonstrated in the detailed tracking of the dominant US clone as it diverges into a series of minor variants (Prospero et al. 2007). This US clone was previously characterized by amplified fragment length polymorphisms (AFLP, Ivors et al. 2004) and SSR markers (Prospero et al. 2004; Ivors et al. 2006) and comprises isolates of only one mating type. An overwhelming predominance of the A2 type in the USA and the A1 mating type in Europe appears to be a fortunate outcome of the tight genetic bottlenecks associated with each introduction. Very low frequencies of the opposite mating type have now been found in US and EU populations and, despite apparent obstacles to mating between these lineages (Brasier & Kirk 2004), there is concern that the pathogen's full evolutionary potential might be realized via sexual recombination. Evidence of a markedly different second US lineage of P. ramorum (Ivors et al. 2006) in Washington State nurseries has again raised concerns about the international trade in plants as a means of disseminating new variants of the pathogen.

Prospero et al. (2007) found no evidence of either recombination or the second US lineage but their high-resolution fingerprinting of the clone did highlight some important details on the Oregon population. First, the distinction between forest and nursery populations strongly supports their independent origins. Isolates from forest outbreaks sampled over four years (2001–2004) were defined by a single dominant genotype (60–70% of the population) that was gradually diversifying over time. Such a pattern was consistent with a trickle of ‘escapees’ from the eradication procedures. Conversely, no single genotype dominated in the nursery outbreaks and no common genotypes were found between the two sampling years (2003–2004). This apparent absence of a ‘resident population’ in the 15 nurseries sampled suggests the eradication procedures are successful but may also reflect the diverse origins and transitory nature of nursery stocks and any resident P. ramorum strains.

This study provides key markers and a detailed insight into the behaviour of P. ramorum in the field but many questions and challenges remain. It would be helpful to know the rate at which new SSR alleles emerge and the likelihood of the independent emergence of identical genotypes (i.e. homoplasy). The application of these markers across larger, more tightly defined sets of samples of US and EU populations will be important, especially if this can be linked to detailed mapping data (e.g. http://www.oakmapper.org). Another goal should be to relate the genotypic data with phenotypic or adaptive traits to build into predictive models of disease spread. Finally, are there lessons to be learnt from the prior P. infestans migrations? The steady ‘clicking’ of Muller's ratchet marked the decline of the dominant P. infestans clonal lineage (Goodwin et al. 1994) and their replacement with fitter imported strains resulting in a rebound of the potato blight problem. This is surely a compelling reason to maintain the state of high alert to prevent the introduction or establishment of new P. ramorum lineages, and indeed the introduction of other destructive pests and pathogens.

References

  1. Top of page
  2. Abstract
  3. References
  • Brasier CM, Kirk S (2004) Production of gametangia by Phytophthora ramorum. Mycological Research, 108, 823827.
  • Gómez-Alpizar L, Carbone I, Ristaino JB (2007) An Andean origin of Phytophthora infestans inferred from mitochondrial and nuclear gene genealogies. Proceedings of the National Academy of Sciences, USA, 104, 33063311.
  • Goodwin SB, Cohen BA, Deahl KL, Fry WE (1994) Migration from northern Mexico as the probable cause of recent genetic changes in populations of Phytophthora infestans in the United States and Canada. Phytopathology, 84, 553558.
  • Ivors KL, Hayden KJ, Bonants PJM, Rizzo DM, Garbelotto M (2004) AFLP and phylogenetic analysis of North American and European populations of Phytophthora ramorum. Mycological Research, 108, 378392.
  • Ivors KL, Garbelotto M, Vries IDE et al . (2006) Microsatellite markers identify three lineages of Phytophthora ramorum in US nurseries, yet single lineages in US forest and European nursery populations. Molecular Ecology, 15, 14931505.
  • Large EC (1940) Advance of the fungi. Jonathan Cape, London.
  • Prospero S, Black JA, Winton LM (2004) Isolation and characterization of microsatellite markers in Phytophthora ramorum, the causal agent of sudden oak death. Molecular Ecology Notes, 4, 672674.
  • Prospero S, Hansen EM, Grünwald NJ, Winton LM (2007) Population dynamics of the Sudden Oak Death pathogen Phytophthora ramorum in Oregon from 2001 to 2004. Molecular Ecology, 16, 29582973.
  • Werres S, Marwitz R, Man In't Veld WA et al . (2001) Phytophthora ramorum sp. nov., a new pathogen on Rhododendron and Viburnum. Mycological Research, 105, 11551165.

David Cooke is a plant pathologist who has extensive experience of using molecular techniques to examine the phylogenetics, speciation processes and the populaton biology of a range of Phytophthora responsible for serious disease problems in agriculture, horticulture and natural ecosystems. He was recently the technical chair of a European project that has developed a comprehensive isolate database to catalogue diversity in the European populations of the potato late blight pathogen, Phytophthora infestans (http://www.eucablight.org).