PHYTOPHTHORA ramorum dropped off the radar, but any hope the disease had burned itself out through depletion of its primary larch tree host was recently dashed.

According to the Forestry Commission’s (FC) Phytophthora ramorum in larch UK Situation Report 01 June 2019, 2018/19 saw a significant upsurge in the number of SPHNs (Statutory Plant Health Notices) issued for felling infected larch, and significantly more compared with the previous peak period of 2013/14. Total SPHNs issued for England, Scotland and Wales in 2013/14 was 550 but 2018/19 saw 850 SPHNs issued, exceeding the 800 issued over the whole five-year period between 2013/14 and 2018/19. These figures were taken from bar charts in the report and as such may not be correct to the last SPHN.

The situation in Scotland is understated due to the Plant Health (Forestry) (Phytophthora ramorum Management Zone) (Scotland) Order 2014 which took effect on 5 June, 2014. Data presented for Scotland since then does not include any suspect or confirmed infections within the South West Scotland Management Zone because SPHNs have not been issued in this zone since that date. The working presumption is most of the 5,000–6,000 ha of larch within the zone is already infected or will be infected in the near future, says the report.

The pathogen is pushing out from the South West Scotland Management Zone with well over 100 SPHNs issued in 2018/19 in areas around the zone, and almost as many again up through western Scotland and into the Grampians.

Likewise for Wales, where the pathogen is fanning out from the Phytophthora ramorum Disease Zone in South Wales into Mid Wales and North Wales, with over 75 SPHNs issued in these areas for 2018/19. The disease remains active in South West England despite a history there going back to 2009. New disease hotspots simmer in north Lancashire and Cumbria.

So, how much larch remains and how much has been lost to the disease so far? The total area of larch in England and Wales subject to felling (or other treatment) following issue of SPHNs since 2010/11 stands at around 19,700 ha. The 2,600 ha quoted for Scotland appears greatly understated, due to no data being recorded in the South West Scotland Management Zone since
5 June, 2014. The estimated volumes of marketed Phytophthora-affected larch timber covered by SPHNs (England only) over this period are 680,000m³.

Prior to discovery of the disease on Japanese larch in South West England in 2009, larch occupied about 154,000 ha (five per cent) of total woodland area across mainland UK, according to Forest Research. The National Forest Inventory shows 111,300 ha of larch woodland at 31 March, 2016, although Forest Research says this would have included areas of larch scheduled for felling due to the presence of P. ramorum.

Following identification of P. ramorum in 2009 on Japanese larch in South West England, the FC published a map of the mainland UK divided into three disease-risk zones. Zone 1, the highest-risk zone, broadly covered South West England, Wales, North-West England and western Scotland.

Zone 3, the lowest-risk zone and broadly covering the eastern side of England north of the River Thames, the East Midlands and eastern Scotland, has so far recorded relatively little disease on larch. In fact, when Scotland is completely excluded from Zone 3, the remainder covering England has apparently not seen a single SPHN. The disease is active in these areas of England, but apparently not on larch. For example, Phytophthora ramorum was found on rhododendron in Epping Forest (Essex) in 2016, though according to the map, no SPHNs have ever been issued for
P. ramorum on larch in Essex.

The FC’s Risk Zone Map appears to have been drawn up on the basis of prevailing climate and weather patterns and their suitability for Phytophthora pathogen activity, and it appears unlikely that larch distribution was included as an input. The amount and distribution of land area down to larch in England does not appear to have been historically differentiated into Japanese, European and hybrid larch.

David Gwillam, owner of Prees Heath Forest Nurseries in Shropshire, has been selling trees for forestry planting over the last 40 years.

“In my experience, relatively little larch was purchased for planting in those areas of England covered by Zone 3 which is designated the lowest disease-risk zone,” said David.

It would appear the dearth of confirmed P. ramorum outbreaks on larch in these English areas of Zone 3 could have as much to do with relatively few larch plantings, as the prevailing drier conditions are generally less conducive to this pathogen and disease.

There appear to be significant differences in the speed with which diseased larch is officially dealt with across the three countries. Estimated areas of larch showing sufficient evidence of infection and awaiting SPHNs to be issued, as of 1 March 2019, was 645 ha, 118 ha and 0 ha for Wales, England and Scotland, respectively.


Forest Research (2019), Ramorum disease (Phytophthora ramorum)

Sketchley, R. (2019) Phytophthora ramorum in larch UK Situation Report 01 June 2019. Forestry Commission England, 7 pages.