Disease

P. ramorum threatens the survival of tanoak in western parts of the tree’s range and is reducing populations of coast live oak dramatically.  By 2014, between 30-45 million trees in California and Oregon were killed. The disease has also resulted in wildlife habitat loss, changes in ecosystem services, and increased fire risk in affected forests. While the NA1 clonal lineage of P. ramorum is most prevalent in California and Oregon forests, the EU1 clonal lineage has also been detected in Curry County, Oregon forests since 2015. There is grave concern that the EU1 lineage, generally shown to be more aggressive than the NA1 lineage in laboratory trials, poses a risk to Douglas-fir, a major timber species. Large areas of U.S. southeastern forests are also considered at risk for the disease (summarized in Kliejunas, 2010). An epidemic is occurring on Japanese larch in the UK where more than  10,000 ha of timber plantations have been affected (King et al. 2015). Heathlands, oak and beech forests, evergreen oak woodlands and laurel forests in Europe are also at risk (Sansford et al. 2009). P. ramorum spreads locally through rain splash of sporangia that form abundantly on the foliage of certain hosts, for example, California bay laurel, Japanese larch, Rhododendron ponticum, or on twigs of tanoak.  Long distance aerial dispersal of sporangia is likely windborne (Hansen et al. 2008). Although the pathogen does not appear to infect roots, soilborne chlamydospores can survive long periods (Fichtner et al. 2009) and give rise to new sporangia that are splashed or carried to infect above-ground parts of plants. P. ramorum has also been baited from forest streams in California and Oregon and from waterways near several infested nurseries.