Phytophthora ramorum and Phytophthora kernoviae in Scotland: status report

This paper sets out the history of these diseases in Scotland, the current situation and the actions being taken to control them.

3. Current situation in Scotland

The following information provides an update on the increased activity introduced by Scottish Government in 2010, along with a brief overview of early developments following the first finding in Scotland in 2002:

Nursery Trade (includes nurseries and garden centres):

In 2010, the Scottish Government committed to inspect all Scottish nurseries growing susceptible species 3 times each year. Those considered to present a higher risk, due to previous infections or proximity to other outbreaks, would be inspected 4 times. From 2012, the number of inspections was reduced to 2, plus 1 additional if deemed at risk.

Table 1, below, provides details of P. ramorum and P. kernoviae findings in the nursery trade. Since 2003 the nursery trade has been largely free from outbreaks with occasional interceptions on non-Scottish material making up the majority of cases. A total of 52 outbreaks/interceptions have been made in Scotland from June 2002 to December 2014.

  • Outbreaks in nurseries, garden centres and newly landscaped sites are usually restricted to specific consignments and can be eradicated within a few months, by destruction of infected plants and quarantine of susceptible species;
  • There has been one finding of P. kernoviae in nursery stock in Scotland;
  • Where an infection is found at a nursery, it is treated according to the EU requirements and the area where infected plants were grown or stored must be cleaned and disinfected;
  • Nurseries which experience an outbreak are likely to suffer a significant financial impact from the destruction of infected stock and quarantine of other plants which may miss their market. However, in most cases it is reasonably straightforward to arrange for destruction and for disinfection of the growing area. Managing pests and diseases is a regular activity for traders, but biosecurity advice is provided along with other standard plant hygiene measures which should prevent spread to other lots.
  • Garden centres are also routinely monitored and those handling host plants are particularly targeted.

Table 1. Number of cases of P. ramorum and P. kernoviae in nursery trade (2002-2014)

Year 02 03 04 05 06 07 08 09 10 11 12 13 14 Total
Outbreaks and
interceptions of
P. ramorum
P. kernoviae
in trade
18 2 5 2 0 5 6 0 0 3 2 4 5 52

Established gardens and public green spaces:

Many ornamental shrubs in heritage and botanic gardens are susceptible to P. ramorum and P. kernoviae. As such, Scottish Government has undertaken annual surveys at gardens and landscaped sites since 2003. There was a particular focus on gardens open to the public and those known to have collections of susceptible plants, and the survey was weighted towards the west of the country which climate modelling shows to be the area at the highest risk. However, a sample of gardens in central and eastern areas were also included.

The first finding of P. ramorum in an established garden in Scotland was in September 2007, followed by P. kernoviae in another garden in January 2008. In 2010 the Scottish Government committed to annually survey 100 established gardens and public green spaces (such as parks and the grounds of public buildings). Despite the increase in surveillance, the number of new outbreak areas detected per year has remained relatively consistent (see Table 2 below). In light of this evidence the Scottish Government has revised the annual surveillance commitment and from 2014 will undertake a survey of 50 gardens and landscape sites throughout Scotland as part of annual monitoring activities. This will be reviewed annually.

Figure 1: Outbreaks of P. ramorum and P. kernoviae in gardens and landscaped sites

Outbreaks of P. ramorum and P. kernoviae in gardens and landscaped sites

Table 2. New outbreaks areas since 2007

Year New outbreaks P. ramorum P. kernoviae Total outbreaks
2007 4 4 0 4 (62)
2008 6 4 2 10 (52)
2009 8 7 1 18 (31)
2010 3 2 1 21 (101)
2011 7 6 1 28 (100)
2012 9 8 1 37 (100)
2013 9 9 0 46 (108)
2014 3 3 0 49 (50)

Note: The number of sites surveyed per year are shown in brackets

Overall position

By December 2014, 45 distinct active 'outbreak areas' had been identified in Scotland, covering 66 premises ranging from large country house grounds and woodland to individual domestic gardens. 47 premises are infected with P. ramorum, 10 with P. kernoviae and 9 with both.

Action taken

Where an infection is found in an established garden, the following measures are taken:

  • all infected plants must be destroyed;
  • other host plants within 2 m radius of infected plants must also be destroyed;
  • restrictions are placed on any plant material being moved from the site. Plant sales may be allowed to continue provided there is strict separation between the nursery and sales area and the rest of the garden;
  • conditions are imposed to limit the risk of spread, including removal of plant debris from paths, restrictions on access if appropriate, and disinfection of tools, footwear and equipment;
  • inspectors revisit the site monthly, monitoring the garden for any new infection and checking that requirements are complied with. If three inspections pass without new infection being found, the frequency is reduced to quarterly inspections, and after three clear quarterly inspections it falls to one inspection annually. After three annual inspections without new infection being found, the outbreak will be considered 'non active' and statutory controls are lifted from the site;
  • Scottish Government was pleased to declare 16 sites had moved to a 'non-active' in the end of December 2014. Yearly monitoring of these premises is still undertaken;
  • A survey of 1.5 km radius around the garden is carried out as soon as possible after the initial finding to check for any wider infection. The Forestry Commission assists in this survey if there is any woodland in the surveyed area. The survey is not repeated, but inspectors will informally monitor other premises if the infected area is close to the boundary of the garden. Additional infection sites have been found in 8 of the 45 existing outbreak areas.

Removal of infected ornamental plants may be physically straightforward, however it is recognised that it can have severe impacts on valuable collections of specimens or historic garden and landscape designs. There are also economic effects from reduced visitor numbers if the garden is significantly changed, and from the need to re-plan and re-plant. Non-symptomatic plants around the infected plant are also normally required to be destroyed to prevent spread, however in a carefully managed garden, vigilance for symptoms and good practice in plant hygiene may be permitted as an alternative.

Heathland/wider environment

In 2010 the Scottish Government committed to annually survey heathland/wider environment sites to check whether P. ramorum or P. kernoviae were present in the wider environment where they have the potential to have a negative impact on the important native habitats. This activity would be carried out by Scotland's Environment and Rural Services ( SEARS) partners, specifically Scottish Environment Protection Agency ( SEPA) and Scottish Natural Heritage ( SNH). No infections have been found, see Table 3 and Figure 2.

Table 3 Surveillance of wider environment sites

Year Number of sites visually inspected Water baiting
2010 53 n/a
2011 107 n/a
2012 19 45 samples (3 positives)
2013 25 17 samples (0 positives)
2014 38 n/a

In addition to inspection of plants for infection, baiting tests in river water were undertaken in late 2012 and early 2013. Locations were sampled focussing on areas identified as high risk for P. ramorum. Two positives were found in water samples in the vicinity of larch outbreaks in Galloway, south west Scotland and the pathogen was also found in one stream in Renfrewshire. Follow up inspections by Forestry Commission Scotland in this area did not reveal any infected plants. A number of other Phytophthora species were detected in the samples, with more diversity in south west than in north east Scotland.

Surveillance in 2008 also found that Vaccinium myrtillus (blaeberry) located in a broad leaf woodland on Arran was infected with P. kernoviae. Control measures put in place have prevented further spread to other hosts and the area is now viewed as non-active. In 2013, P. ramorum was found on Vaccinium in Galloway in an infected larch plantation.

Figure 2. SEARS survey for Phytophthora in heathland/wider environment locations by year;
a) 2010, b) 2011, c) 2012, d) 2013, e) 2014

SEARS survey for Phytophthora in heathland/wider environment locations by year 2010

SEARS survey for Phytophthora in heathland/wider environment locations by year 2011

SEARS survey for Phytophthora in heathland/wider environment locations by year 2012

SEARS survey for Phytophthora in heathland/wider environment locations by year 2013

SEARS survey for Phytophthora in heathland/wider environment locations by year 2014

Woodlands and forestry

In 2009 the pathogen was detected on larch trees in south west England, the first time in the world that a commercial conifer species had become infected. In late 2010 1.5 ha of Japanese larch was found to have been infected on the Craignish peninsula south of Oban. Subsequently, several new sites were detected through FCS's bi-annual aerial surveillance programme, primarily in the west of Scotland. Wet and windy conditions in 2012 in the south west of Scotland were extremely conducive to the spread of P. ramorum and gave rise to a major increase in the rate, area and severity of outbreaks in the Galloway area in 2013, with some 5000-6000 ha now thought to be harbouring infection in that area. Subsequently, it was considered to be no longer feasible or appropriate to continue with mandatory felling/killing of all infected larch stands in this area.

Following much drier conditions in the summer of 2013, surveillance flights over Scotland confirmed limited expansion of the disease in 2014. Outside of the Galloway Management Zone, there were just 20 detections (at 13 locations), totalling 50 ha, all within 10 km of previously known infections. Between May and September 2014, 15 flight days were completed, covering 77,000 ha of larch in a forestry setting, which represents approximately 93% of the larch resource in Scotland. From this nearly 400 sites were identified for field survey. The follow-up investigations confirmed 21 positives, resulting in 14 new Statutory Plant Health Notices ( SPHNs).

Given that the summer of 2014 was also relatively dry, a similar pattern of limited disease expansion on larch is expected for 2015, but this will need to be validated during the aerial surveillance programme.

Figure 3: Aerial surveillance routes (May - September 2014), shown in green

Aerial surveillance routes (May – September 2014), shown in green

Within the core infected area, the Management Zone (formerly known as the Red Zone), it is now the case that the only statutory plant health controls normally applied to larch relate to the movement of timber, bark and other susceptible material. Where such material is moved outside the Zone it can only be sent to approved facilities and a record of that movement needs to be kept - as provided in the Plant Health (Forestry) ( Phytophthora ramorum Management Zone) (Scotland) Order 2014. However, FCS retains the ability to issue statutory notices requiring the felling/killing of infected larch stands in this area where such actions are felt to be necessary.

Figure 4: Known distribution of P. ramorum infection on larch in Scotland (1 April 2015)

Known distribution of P. ramorum infection on larch in Scotland (1 April 2015)

In the rest of Scotland, to help reduce the extent, rate of spread and severity of P. ramorum infections, statutory plant health notices will continue to be issued and will now require the felling of infected stands of larch and all surrounding larch within a 250m radius. The movement and processing of timber and other susceptible material from such sites is regulated through a licensing system operated by the Forestry Commission, which includes the application of biosecurity measures.

Although P. ramorum infections generally do not affect the quality or end use of the timber, they often still have significant financial impacts because trees may need to be felled before their planned removal date and, in some cases, well before they are fully mature (in which case they may have less, or sometimes no, commercial value). Infections also have implications for the cost of access to and felling/killing of infected stands and there are additional costs for biosecurity, transport and utilisation of susceptible material. A further issue is that P. ramorum and P. kernoviae have such a wide host range that options for replanting may be constrained on some site types.

Findings of P. ramorum and P. kernoviae on other tree species in Scotland have been very rare and invariably associated with immediately adjacent, high inoculum loadings. Of the broadleaved tree species in Scotland, individual oak and beech trees have been found to have become infected (as non-sporulating hosts), whereas a slightly wider range of conifers (including Douglas fir, Western hemlock, Grand fir and even Sitka spruce) are known to be occasional, non-sporulating hosts as part of the collateral damage associated with adjacent larch infections. Despite such trees being non-sporulating hosts, statutory action requiring their felling/killing remains a precautionary option.

Wider prophylactic clearance

R. ponticum is involved in many garden and wider environment outbreaks. Removal of rhododendron to control P. ramorum or P. kernoviae is normally beneficial to the habitat, enabling native species to re-colonise the area. It is, however, physically difficult and expensive; rhododendron often grows densely on steep and inaccessible terrain, and is persistent, requiring regrowth to be treated for at least 3 years after removal. There may also be other consequences to consider, for example where Rhododendron ponticum is planted to protect less hardy plants from wind or sea-spray, or where its removal may impact on short-term soil stability on steep hillsides.

In December/January 2010, Forestry Commission Scotland undertook a desk-based assessment of the cost implications of prophylactic clearance of all potential sporulating host plants within a 100m radius of each known P. ramorum and P. kernoviae infection in Scotland. This suggested a total, direct cost of some £500,000. The epidemiological impact of such work cannot readily be quantified.

At a meeting in April 2010, the Scottish Government Phytophthora Steering Group agreed that in most cases, heightened control and surveillance activity would enable any new infections to be dealt with quickly and effectively. Two outbreaks were identified where the scale of infection on Rhododendron ponticum, coupled with the extent of dense stands of that species, was such that prophylactic clearance of potential sporulating host plants would be a sensible precaution. However, subsequent work by Forestry Commission Scotland to facilitate such clearance through SRDP funding indicated that this was not regarded by land managers as an effective support measure for such circumstances. The forthcoming FCS/ SNH strategy for controlling invasive rhododendron in Scotland will seek to address this and other funding issues relating to the management of R. ponticum.


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