What Risk Does <i>Phytophthora ramorum</i> Pose to Sitka Spruce ( <i>Picea sitchensis</i> ) and Other Conifers in Britain?

ABSTRACT Larch ( Larix ) and, to a lesser extent, rhododendron ( Rhododendron ponticum ) have emerged as key drivers of the Phytophthora ramorum epidemic in Britain because of the copious numbers of sporangia that are released from their foliage following infection; the inoculum then initiates stem and foliar infections on nearby broadleaf and conifer species. This has raised concerns that other conifer species could play a similar role to larch, with most concern centred on Sitka spruce which currently comprises &gt; 50% of all commercial conifer plantations in Britain. To address this, we assessed the potential of the EU1 lineage of P. ramorum to sporulate on foliage of conifers already known to have some susceptibility (Sitka spruce ( Picea sitchensis ), grand fir ( Abies gradis ), Douglas fir ( Pseudotsuga menziesii ), western hemlock ( Tsuga heteophyllum ) and coastal redwood ( Sequoia sempervirens )). Our findings suggested these conifers are much less effective at sustaining sporulation (means ranging from ~16 to ~49 sporangia per cm 2 of needle) compared with larch (nearly 600 per cm 2 ). Lesions formed by both EU1 and EU2 lineages of P. ramorum in Sitka spruce bark were significantly smaller than those in larch, indicating that spruce is a less susceptible host. However, substantial zoospore concentrations (50,000 to 500,000 mL −1 ) were required to cause infections through intact bark of both larch and spruce. Outcomes of these comparisons are discussed in the context of the frequency with which larch and Sitka spruce were found to be naturally infected during surveys of P. ramorum in Britain. Overall, we conclude that in contrast to larch, Sitka spruce and several other conifers grown in Britain do not have potential to sustain an epidemic of P. ramorum , although they may possibly act as low‐level reservoirs of infection.