Novel introductions and epidemic dynamics of the sudden oak death pathogen Phytophthora ramorum in Oregon forests

Authors: CARLESON, N - Oregon State University; DANIELS, H - Oregon State University; Larsen, Meredith; SUTTON, W - Oregon State University; REESER, P - Oregon State University; KANASKIE, A - Oregon Department Of Forestry; HANSEN, E - Oregon State University; NAVARRO, S - Oregon Department Of Forestry; LEBOLDUS, J - Oregon State University; Grunwald, Niklaus - Nik

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2020
Publication Date: 3/9/2021
Citation: Carleson, N.C., Daniels, H.A., Larsen, M.M., Sutton, W., Reeser, P.W., Kanaskie, A., Hansen, E.M., Navarro, S.M., Leboldus, J.M., Grunwald, N.J. 2021. Novel introductions and epidemic dynamics of the sudden oak death pathogen Phytophthora ramorum in Oregon forests. Phytopathology. 111(4):731-740. https://doi.org/10.1094/PHYTO-05-20-0164-R.
DOI: https://doi.org/10.1094/PHYTO-05-20-0164-R

Interpretive Summary: Sudden oak death caused by the water mold Phytophthora ramorum has been actively managed in Oregon since the early 2000s. To date, this epidemic has been driven mostly by the NA1 clone of P. ramorum, but an outbreak of the new EU1 clone has recently emerged. Here we contrast the genetic diversity of the NA1 outbreak first reported in 2001 to the outbreak of the EU1 clone first detected in 2015. We tested if any of the clones were introduced more than once. Infested regions of the forest were sampled between 2013-2018 including 903 strains. Genetic diversity was characterized using genetic markers suitable for genotyping. Most genotypes observed were transient, with 272 of 358 unique genotypes emerging one year and disappearing the next. Diversity of EU1 was very low suggesting a single EU1 introduction. In contrast, the older NA1 populations were more diverse and more widely spread. Statistical analysis supported two to four independent NA1 introductions. Comparing these epidemics provides novel insights into patterns of emergence of clonal pathogens in forest ecosystems.

Technical Abstract: Sudden oak death caused by Phytophthora ramorum has been actively managed in Oregon since the early 2000s. To date, this epidemic has been driven mostly by the NA1 clonal lineage of P. ramorum, but an outbreak of the EU1 lineage has recently emerged. Here we contrast the population dynamics of the NA1 outbreak first reported in 2001 to the outbreak of the EU1 lineage first detected in 2015. We tested if any of the lineages were introduced more than once. Infested regions of the forest were sampled between 2013-2018 (n = 903) and strains were genotyped at 15 microsatellite loci. Most genotypes observed were transient, with 272 of 358 unique genotypes emerging one year and disappearing the next. Diversity of EU1 was very low and isolates were spatially clustered (< 8 km apart), suggesting a single EU1 introduction. Some forest isolates are genetically similar to isolates collected from a local nursery in 2012, suggesting introduction of EU1 from this nursery or simultaneous introduction to both the nursery and latently into the forest. In contrast, the older NA1 populations were more polymorphic and spread over 30 km2. Principal component analysis supported two to four independent NA1 introductions. The NA1 and EU1 epidemics infest the same area but show disparate demographics owing to initial introductions of the lineages spaced 10 years apart. Comparing these epidemics provides novel insights into patterns of emergence of clonal pathogens in forest ecosystems.