Population Dynamics of the Sudden Oak Death Pathogen Phytophthora ramorum in Oregon from 2001 to 2004

Authors: PROSPERO, S - UMR BIOGECO; HANSEN, E - OREGON STATE UNIVERSITY; Grunwald, Niklaus - Nik; Winton, Loretta

Submitted to: Molecular Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2007
Publication Date: 7/6/2007
Citation: Prospero, S., Hansen, E.M., Grunwald, N.J., Winton, L.M. 2007. Population dynamics of the sudden oak death pathogen Phytophthora ramorum in Oregon from 2001 to 2004. Molecular Ecology. 16:2958-2973.

Interpretive Summary: Phytophthora ramorum is an emerging plant pathogen in Curry County forests in southwestern Oregon. Moreover, since 2003 it has been repeatedly isolated from plants in Oregon nurseries. In this, study we analyzed the genetic diversity of the population of this pathogen in Oregon from 2001 to 2004 by using microsatellites. Microsatellites are a common molecular tool used to characterize the genetic diversity and structure of populations of any organism. A total of 323 isolates (272 from the infested forest and 51 from nurseries) were screened at 10 microsatellite loci. Twenty-four multilocus genotypes were identified from the forest. Genotypic diversities were low among the 2001 to 2004 forest populations. In contrast, the nursery population was composed of 11 genotypes. No dominant genotype was observed and only two nursery genotypes were also found in the forest. Significant differentiation, showing that clear differences in the genetic structure of populations, was detected between nursery and forest populations. One nursery genotype belonged to the European lineage of P. ramorum. No evidence of sexual recombination between isolates belonging to the European (A1 mating type) or North American (A2 mating type) clonal lineages was observed. All A2 isolates screened belonged to the same clonal lineage. The low population diversity of P. ramorum in southwestern Oregon supports the hypothesis that this pathogen is not native to Oregon. The nursery infestation is not caused by the genotypes observed in Curry County, but likely resulted through introduction of novel genotypes from nurseries located outside Oregon. This highlights the continued importance of sanitation and quarantine in nurseries to prevent further introduction and spread of P. ramorum.

Technical Abstract: Phytophthora ramorum (Oomycetes) is an emerging plant pathogen in forests in southwestern Oregon (Curry County). Moreover, since 2003 it has been repeatedly isolated from plants in Oregon nurseries. In this study, we analyzed the genetic diversity of the P. ramorum population in Oregon from 2001 to 2004 by using microsatellites. A total of 323 isolates (272 from the infested forest and 51 from nurseries) were screened at 10 microsatellite loci. Twenty-four multilocus genotypes were identified from the forest. In all sampling years, 60-70% of the isolates belonged to the same genotype. Genotypic and gene diversities were low and no significant differentiation was detected among the 2001 to 2004 populations. The nursery population was composed of 11 genotypes. No dominant genotype was observed and only two nursery genotypes were also found in the forest. Significant differentiation was detected between nursery and forest populations. One nursery genotype belonged to the European lineage of P. ramorum. No evidence of sexual recombination between isolates belonging to the European (A1 mating type) or North American (A2 mating type) clonal lineages was observed. All A2 isolates screened belonged to the same clonal lineage. The low population diversity of P. ramorum in southwestern Oregon supports the hypothesis that this pathogen is not native to Oregon. The nursery infestation is not caused by the genotypes observed in Curry County, but likely resulted through introduction of novel genotypes from nurseries located outside Oregon. This highlights the continued importance of sanitation and quarantine in nurseries to prevent further introduction and spread of P. ramorum.