EXOTIC, EMERGING, RE-EMERGING, AND INVASIVE PLANT DISEASES OF HORTICULTURAL CROPS
Location: Horticultural Crops Research
Title: First Report of the European Lineage of Phytophthora ramorum in a California Nursery
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 30, 2007
Publication Date: January 15, 2008
Citation: Grunwald, N.J., Goss, E.M., Larsen, M.M., Press, C.M., Mcdonald, V.T., Blomquist, C.L., Thomas, S.L. 2008. First report of the European lineage of Phytophthora ramorum in a California nursery. Plant Disease. 92:314.
Interpretive Summary: The pathogen Phytophthora ramorum causes disease of many ornamental and forest plants, including Sudden Oak Death on oaks. Phytophthora ramorum is an exotic pathogen recently found in nurseries. In the United States, P. ramorum currently exists as three distinct clones. We found coexistence of two clones of opposite mating types in a California retail nursery. Presence of opposite mating types in the same retail nursery suggests the potential for sexual reproduction.
Phytophthora ramorum is the causal agent of Sudden Oak Death in California and Oregon forests and Ramorum blight on broad range of host species in wildlands and nurseries. It is thought to be an introduced pathogen and only three clonal lineages are known. The North American lineage (lineage NA1, mating type A2) is responsible for infections in CA and OR forests. The European lineage (lineage EU1, predominantly A1) is responsible for infections in Europe, but has also been found in nurseries in OR and WA. A third lineage (NA2) has only been isolated in a few instances from nurseries in WA and CA. In June 2006, P. ramorum was isolated from diseased Viburnum tinus, Osmanthus heterophyllus, and O. fragrans cultivars from a Humboldt County retail nursery in northern California. Isolates were genotyped and placed into a clonal lineage using microsatellite markers developed for P. ramorum. Three out of four isolates were found to belong to genotype EU1. The fourth isolate, obtained from O. fragrans, belonged to genotype NA1. Genotyping was repeated using an independent genomic DNA extraction. Two EU1 isolates and the single NA1 isolate were tested for mating type and found to be of A1, A1 and A2 mating type, respectively. The coexistence of A1 and A2 mating types in the same retail nursery suggests the potential for sexual reproduction although to date sexual reproduction has not been documented in P. ramorum. The CA retail nursery infestation highlights the potential for unintentional transport of infested host nursery stock.