In 2008, Plant Protection and Quarantine (PPQ) Plant Epidemiology and Risk Analysis Laboratory (now Plant Pest Risk Analysis) conducted a risk assessment for Phytophthora ramorum Werres, De Cock & Man in’t Veld and concluded P. ramorum was a high-risk pathogen for large areas of the United States. However, as of 2023 in the United States, P. ramorum is still only established and causing disease in the coastal areas of California and Oregon despite repeated nursery and stream detections throughout the country. In this document, we revised our 2008 assessment of the risk P. ramorum poses to areas of the United States where it is not present using the latest scientific information on its biology, ecology, and epidemiology.

Outside of California and Oregon forests, there is an asynchrony between the infective stage of P. ramorum and the susceptible stage of the host complex when environmental conditions are favorable for infection, disease development, and spread. Three interrelated factors are likely causing this asynchrony and making the occurrence of diseases caused by P. ramorum unlikely:

1. Environmental stress, such as heat stress, decreases inoculum survival of P. ramorum.
2. Inoculum does not build up in sufficient amount to produce significant disease.
3. The infectious stage of the P. ramorum lifecycle, (i.e., zoospore production) does not overlap with the susceptible stage of the host or host complex for sufficient time, reducing the likelihood of infection and disease development.

Thus, while P. ramorum may survive in the environment, the necessary conditions for the development of ramorum blight, ramorum shoot dieback, and sudden oak death in forests are not occurring, and consequently hosts are not symptomatic—at least not on a noticeable scale. While it is possible that repeated incursions of the pathogen, or changes in climate conditions, in an area could increase inoculum pressure enough to cause infection and disease under the right conditions, twenty years of observations with this pathogen and numerous movements of infected plant material outside of California and Oregon, lead us to conclude that this is unlikely. Therefore, unless conditions change, P. ramorum is unlikely to pose a high risk to the United States outside of forests in California and Oregon.

There are three important sources of uncertainty in this assessment. First, there is still a lot unknown about the competency and susceptibility of eastern U.S. hosts and it is still unclear if there is synchrony between inoculum production and times of host susceptibility. Second, modeling climatic suitability remains challenging. Some climatic factors important for disease development cannot be reliably modeled for forest conditions. Third, a host range expansion due to the introduction of new clonal lineages or the emergence of new lineages due to sexual recombination may increase the adaptability of P. ramorum in the United States and potentially alter the consequences of introduction. The jump of P. ramorum to larch (Larix spp.) in Europe in 2009, and the introduction of the EU1 clonal lineage to the United States, which occurred around 2016, suggests a host range expansion could affect U.S. conifers or other plants. If EU1 expanded in range, or if other lineages were to establish, the chances for sexual recombination may increase, which could result in changes in the biology and epidemiology of this pathogen.