THE first mature tree found with Phytophthora ramorum in the UK was a 100-year-old southern red oak (Quercus falcata), in November 2003. The fungus-like pathogen had entered the country as the EU1 genetic lineage on imported Rhododendron and Viburnum plants from the Netherlands and Germany, having existed there since the mid 1990s as an unidentified Phytophthora. 

P. ramorum swiftly moved into the wider environment to infect a range of plant species including Rhododendron ponticum, its large evergreen leaves providing year-round infection and sporulation templates for P. ramorum and a ‘jump-off’ point for infection of other plant species. One of these was Japanese larch (Larix kaempferi), found with shoot and needle blight and resinous bark cankers in south-west England in 2009. Pathogen and disease have subsequently spread like wildfire to destroy any prospects for Japanese larch as a long-term timber resource.

Concern then turned to other conifers, with foresters especially fearful about Sitka spruce (Picea sitchensis). Fears were realised in 2017, when P. ramorum was found on Sitka spruce in south-west Scotland, on a group of trees in Galloway and a single tree near the village of Ae north of Dumfries.

The Forestry Commission reported the findings, but they were not widely publicised until March 2018, when an article in The Sunday Herald asked if P. ramorum was poised to infect Sitka spruce in a substantial way.

FC Scotland and Confor dismissed the suggestion, saying the reports referred to outbreaks on Sitka spruce recorded in 2017 and how these were not the first incidences of P. ramorum on Sitka spruce. Confor told The Sunday Herald how it took comfort in the “historic resilience” of Sitka spruce to P. ramorum, while FC Scotland said “scientific evidence shows that Sitka spruce is considered to be of very low susceptibility to P. ramorum”, and emphasised the infected trees were adjacent to diseased larch.

However, extensive research conducted some 15 years earlier suggests that assumptions about the resilience of Sitka spruce should be treated with caution.

When P. ramorum first appeared in 2002/2003, DEFRA and the FC immediately embarked on a substantial programme of research. DEFRA-funded Project No. PH0194, ‘Phytophthora ramorum epidemiology: sporulation potential, dispersal, infection and survival’, comprised a range of trials including inoculation of three-year-old tree saplings and mature logs from different species, including Sitka spruce. Susceptibility of three-year-old saplings varied according to season and whether the plant was wounded or not. Under summer conditions and using wounded saplings, Sitka spruce and Douglas fir (Pseudotsuga menziesii) proved to be most susceptible. Wounding was not required for infection of mature logs of Sitka spruce (DEFRA, 2004).

In 2005, scientists at Forest Research carried out leaf inoculation studies on 23 broadleaves and 11 conifers as an indication of tree foliage susceptibility to P. ramorum (Denman et al. 2005). 

A summary of findings for Sitka spruce showed:
• Overall disease incidence (per cent leaf necrosis) was 53 per cent (75 per cent in summer and 31 per cent in winter), placing this conifer in the ‘high’ susceptibility grouping.
• Per cent leaf necrosis for non-wounded and wounded leaves of Sitka spruce was 31 and 75 respectively.
No larix (larch) species were included in the study, which is unfortunate because, if they had been, we might have got an insight into what was to befall the hapless Japanese larch at the hands of P. ramorum four years later in 2009.

So is Sitka spruce safe? Hopefully, yes. It took more than five years for P. ramorum to find its way from Rhododendron ponticum onto Japanese larch and to subsequently infect the conifer in epiphytotic (epidemic) proportions. It is now almost two decades since P. ramorum was first found in the UK. Provided UK plant health authorities promptly fell infected Japanese larch and remove Rhododendron ponticum, whether or not infected, then opportunities for reproduction of the pathogen on its top two sporulation hosts is minimised.

This, in turn, should drastically reduce scope for genetic change in the pathogen population in the UK. However, as long as we are bound by Brussels’ D’Artagnan Doctrine (all for one), with essentially unimpeded movement of pests and pathogens between EU (European Union) member states, then arrival of a another genetic lineage of P. ramorum with its own host range/pathogenicity profile remains the biggest threat to Sitka spruce and other commercial conifers in the UK.

In 2011, a second genetic lineage (EU2) for P. ramorum was discovered in south-west Scotland and Northern Ireland.

DEFRA (2004) Phytophthora ramorum epidemiology: sporulation potential, dispersal, infection and survival. Project PH0194. DEFRA (Department for Environment, Food and Rural Affairs), London, UK.
Edwards, R. (2018) ‘Sudden’ Death killer stalking Scottish forests. The Sunday Herald 18th March 2018. 
Denman, S. Kirk, S.A., Brasier, C.M. and Webber, J.F. (2005) In vitro leaf inoculation studies as an indication of tree foliage susceptibility to Phytophthora ramorum in the UK. Plant Pathology (2005) 54, 512–521.