PCR-RFLP markers identify three lineages of the North American and European populations of Phytophthora ramorum

Phytophthora ramorum, causal agent of sudden oak death, has a wide host range and is found in the northern hemisphere. It is thought to be introduced to North America and Europe, but its origin is unknown. It has three major clonal lineages and two mating types. Sexual reproduction can only occur when both mating types are present in the same location. In most cases, these mating types have been restricted to different continents. The European lineage (EU1, mostly A1 mating type) has been consistently found in Europe, and occasionally in North American nurseries. The North American lineages (NA1 and NA2, all A2 mating type) have not been found in Europe at present. All molecular tests currently available for detecting P. ramorum do so at the species level. In tests that use the ITS region, cross-reaction with other closely related species such as Phytophthora hibernalis, Phytophthora foliorum, or Phytophthora lateralis can occur. Regions in the mitochondrial gene Cox1 are different among P. ramorum lineages and mitochondrial genotyping of the North American and European populations seems to be sufficient to differentiate between mating types, since the EU1 lineage is mostly A1 and both NA1 and NA2 lineages are A2. P. ramorum isolates can be identified to lineage using PCR-RFLP of the Cox1 gene, first using Apo1 to separate P. ramorum from other species and EU1 from North American populations, and then Ava1 to distinguish between NA1 and NA2 genotypes. However, P. folio-rum had the same mtDNA genotype as P. ramorum NA1 isolates.