Phytophthora ramorum was first found in Europe in 1993 and has been found in nurseries, horticultural and wild settings in many European countries since then. However, the strains of P. ramorum in Europe (EU1 and EU2), is different from the strain originally introduced into California (strain NA1) (Ivors and others 2006). By some measures, the EU1 strain is more aggressive than the NA1 strain. A third unique strain of P. ramorum was initially known only from nurseries in Washington State and is designated as NA2. This strain has since been found in California and nurseries in British Columbia (table 1-5). A second European strain (EU2) is present on Japanese larch and other hosts (Larix kaempferi (Lam.) Carriere) in the United Kingdom in Northern Ireland and southwest Scotland (King and others 2015). The distribution of these four strains and other genetic data indicate that P. ramorum is an introduced pathogen in both North America and Europe. The native range of P. ramorum is unknown at this time.
Both the European (EU1) and NA2 strains have been found in California nurseries. To date, the EU1 strain has been detected outside of a retail nursery site in Humboldt County. From a disease management standpoint, it is important to avoid widespread introduction of the European strain into California because the European and California strains are of opposite mating types (A1 and A2, respectively). The A2 mating type strain in California forests has been limited to asexual reproduction only. Introduction of the A1 mating type into California forests could allow the pathogen to reproduce sexually and asexually. This could introduce new variability into the P. ramorum population, possibly increasing the pathogen's virulence, host range, and/or adaptation to a wider range of climatic conditions.
Table 1-5— Summary of currently known Phytophthora ramorum strains
| P. ramorum strain | Mating type1 | Description |
| NA1 | A2 | Original strain introduced into CA and OR forests and currently most widely distributed. Also common in US nursery detections. |
| NA2 | A2 | Second strain detected in US nurseries, originally in WA; not closely related to NA1. Also in nurseries in British Columbia, Canada, and CA. |
| EU1 | A1 | Strain originally found in European nurseries and in landscape introductions in Europe. Found in U.S. nurseries, especially in Washington. Has spread from nurseries to waterways in Washington, also found in a river in Humboldt County. |
| EU2 | A1 | Strain identified from Northern Ireland and western Scotland from isolates collected in 2011, with an earliest collection date in 2007. This is the second strain detected from Japanese larch in the United Kingdom |
1Sexual reproduction may be possible if A1 and A2 strains are crossed, but does not occur if only one mating type is present.