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A Rapid Diagnostic Test to Distinguish Between American and European Populations of Phytophthora ramorum

A new devastating disease in the United States, commonly known as Sudden Oak Death, is caused by Phytophthora ramorum. This pathogen, which previously was described attacking species of Rhododendron and Viburnum in Germany and the Netherlands, has established itself in forests on the central coast of California and is killing scores of native oak trees (Lithocarpus densiflora, Quercus agrifolia, Q. kelloggii, and Q. parvula var. shrevei). The phytosanitary authorities in the European Union consider non-European isolates of P. ramorum as a threat to forest trees in Europe. To date, almost all European isolates are mating type A1 while those from California and Oregon are type A2. The occurrence of both mating types in the same region could lead to a population capable of sexual recombination, which could generate a new source of diversity. To prevent contact between these two populations, a rapid, reliable, and discriminating diagnostic test was developed to easily distinguish the two populations. Based on a DNA sequence difference in the mitochondrial Cytochrome c oxidase subunit 1 (Cox1) gene, we developed a single-nucleotide polymorphism (SNP) protocol to distinguish between isolates of P. ramorum originating in Europe and those originating in the United States. A total of 83 isolates of P. ramorum from Europe and 51 isolates from the United States were screened and all isolates could be consistently and correctly allocated to either the European or the U.S. populations using the SNP protocol.