Population structure of Phytophthora plurivora on Rhododendron in Oregon nurseries

Authors: CARLESON, NICK - Oregon State University; FIELAND, VALERIE - Oregon State University; Scagel, Carolyn; Weiland, Gerald - Jerry; Grunwald, Niklaus - Nik

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2019
Publication Date: 5/29/2019
Citation: Carleson, N.C., Fieland, V.J., Scagel, C.F., Weiland, G.E., Grunwald, N.J. 2019. Population structure of Phytophthora plurivora on Rhododendron in Oregon nurseries. Plant Disease. 103(8):1923-1930. https://doi.org/10.1094/PDIS-12-18-2187-RE.
DOI: https://doi.org/10.1094/PDIS-12-18-2187-RE

Interpretive Summary: Phytophthora plurivora is a recently described plant pathogen, formerly recognized as Phytophthora citricola. Recent sampling of Pacific Northwest nurseries frequently encountered this pathogen, and it has been shown to be among the most damaging Phytophthora pathogens on ornamentals. We characterized the population structure of P. plurivora in a survey of four Oregon nurseries in three different counties with focus on Rhododendron hosts. Samples were identified to the species by sequencing. We studied genetic diversity using high throughput sequencing and identifying genetic variation. Overall, population structure suggested presence of one dominant clonal lineage in all nurseries, as well as isolates of cryptic diversity mostly found in one nursery. Within the clonal lineage, there was a broad range of sensitivity to mefenoxam and phosphorous acid. There was also a correlation between sensitivity to the two fungicides. P. plurivora was previously assumed to primarily spread clonally, and the observed low genotypic diversity within and among samples corroborated this hypothesis. These findings provide the first perspective into P. plurivora population dynamics in Pacific Northwest nurseries.

Technical Abstract: Phytophthora plurivora is a recently described plant pathogen, formerly recognized as P. citricola. Recent sampling of Pacific Northwest nurseries frequently encountered this pathogen, and it has been shown to be among the most damaging Phytophthora pathogens on ornamentals. We characterized the population structure of P. plurivora in a survey of four Oregon nurseries across three different counties with focus on Rhododendron hosts. Isolates were identified to the species level by Sanger sequence and/or a PCR-RFLP assay of the internal transcribed spacer (ITS) region. We used genotyping-by-sequencing (GBS) to determine genetic diversity. Variants were called de novo resulting in 284 high-quality variants for 61 isolates after stringent filtering. Based on Fst and AMOVA, populations were moderately differentiated among nurseries. Overall, population structure suggested presence of one dominant clonal lineage in all nurseries, as well as isolates of cryptic diversity mostly found in one nursery. Within the clonal lineage, there was a broad range of sensitivity to mefenoxam and phosphorous acid. There was a correlation between sensitivity to the two fungicides. P. plurivora was previously assumed to primarily spread clonally, and the observed low genotypic diversity within and among isolates corroborated this hypothesis. The broad range of fungicide sensitivity within the P. plurivora population found in PNW nurseries has implications for managing disease caused by this important nursery pathogen. These findings provide the first perspective into P. plurivora population structure and functional diversity in Pacific Northwest nurseries.