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United States Department of Agriculture

Agricultural Research Service

Research Project: EXOTIC, EMERGING, RE-EMERGING, AND INVASIVE PLANT DISEASES OF HORTICULTURAL CROPS

Location: Horticultural Crops Research

Title: Migration Patterns of the Emerging Plant Pathogen Phytophthora ramorum on the West Coast of the United States of America

Authors
item Prospero, S - SWISS FEDERAL INSTITUTE
item GRUNWALD, NIKLAUS
item Winton, Loretta
item Hansen, E - OREGON STATE UNIVERSITY

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 19, 2009
Publication Date: June 1, 2009
Citation: Prospero, S., Grunwald, N.J., Winton, L.M., Hansen, E.M. 2009. Migration patterns of the emerging plant pathogen Phytophthora ramorum on the west coast of the United States of America. Phytopathology. 99:739-749.

Interpretive Summary: The water mold Phytophthora ramorum is the causal agent of sudden oak death and ramorum blight on trees, shrubs, and woody ornamentals in the forests of coastal California and southwestern Oregon and in nurseries of California, Oregon, and Washington. In this study, we investigated the genetic structure of P. ramorum on the west coast of the United States. A total of 576 isolates recovered from 2001 to 2005 were analyzed using molecular markers. Detailed analysis showed a lack of genetic structure among all populations and indicated that most populations had bi-directional migration of P. ramorum strains. Our results suggest that large scale, regional range expansion of P. ramorum within the west coast resulted mostly from the movement over long distances of infected nursery stocks, as previously suggested by trace-back and trace-forward surveys. Consequently, the current P. ramorum population is genetically homogeneous from California to Washington.

Technical Abstract: Phytophthora ramorum (Oomycetes) is the causal agent of sudden oak death and ramorum blight on trees, shrubs, and woody ornamentals in the forests of coastal California and southwestern Oregon and in nurseries of California, Oregon, and Washington. In this study, we investigated the genetic structure of P. ramorum on the west coast of the United States, focusing on population differentiation potentially indicative of gene flow. A total of 576 isolates recovered from 2001 to 2005 were genotyped at ten microsatellite loci. Twenty out of 72 genotypes identified were shared among populations and based on assignment tests only 39 genotypes were attributed to the population from which they apparently originated. Only in the Oregon forest population were the majority of the genotypes assigned correctly. AMOVA showed a lack of genetic structure among all populations and maximum-likelihood based analysis revealed that most populations were well connected bi-directionally. Our results support previous data from trace-back and trace-forward surveys that suggest large scale, regional range expansion of P. ramorum within the west coast resulted from movement over long distances of infected nursery stock.

Last Modified: 7/28/2014
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