Outbreaks of Phytophthora ramorum are still unusual in British gardens. But the recent discovery of cases among a range of trees in south-west England suggests that even sites well away from Rhododendron, the main incubator of the disease, are at risk.
According to Forestry Commission Plant Health Service head Roddie Burgess: "It's quite a cause for concern because it's the first significant finding in trees and a commercially important one in Japanese larch (Larix). Every other site has had Rhododendron present, which we know is the principle sporulator. But the places where Japanese larch were found infected had very little Rhododendron or none at all."
Phytophthora ramorum first came to prominence in the US, where it is known as sudden oak death. Native British oaks are not so vulnerable to the disease - but many more tree and shrub species are and the number is growing.
Moreover, it can be transferred by air or soil and will tolerate a wide range of temperatures and humidity, though it appears to prefer the warmer, wetter west side of the UK. And with no effective treatment, vigilance is called for to prevent further spread.
Outbreaks in trees have so far been comparatively rare, with only around 80 cases since the disease first appeared in the UK in 2003, though these have been on a range of species, principally beech. While the disease causes little more than leaf damage to Rhododendron, the lesions it causes on trunks can kill the tree.
"Once it gets onto beech, that's a dead-end - it can't reproduce further," says Burgess. "But the worry with Japanese larch is that spores are being produced on the foliage. It's not that it's mutating - the samples show it's all "mainstream" P. ramorum. But we also know that it's not come in imported stock because there's no evidence of Japanese larch being infected anywhere else."
What the authorities don't have - and need to properly inform people - is a clear understanding of how the disease spreads, he says. "How far can spores travel? Do infected trees need to be felled? Is there a risk from moving the wood around, with or without the bark? Where can you take the wood and is there a need for bio-security measures at their destination? Do you need to clean the lorries? These questions need to be answered before measures are put in place. We don't want to go off at half-cock or introduce expensive measures that turn out to be unnecessary."
Funding committed to key research
In an indication of the severity and complexity of the problem, in April DEFRA committed £25m to a five-year programme of research into the disease and improved measures to control its spread. Some of this is already being used to try to answer these questions, Burgess adds.
"Infected sites are already receiving £760,000 a year. It's made life easier for us. It's still spreading, albeit slowly, and this could allow us to halt and reverse it."
No treatments are available. Indeed, Defra strongly discourages use of Phytophthora fungicides, saying: "Such fungicides are likely to suppress but not eradicate the pathogens."
Deborah Elton is the Forestry Commission's policy implementation officer in the south west region, where incidences have been most numerous.
"There are three strains of P. ramorum that each behave differently," she says. "There's a lot of science still to be done and we need to have good evidence [to decide how to treat it].
"So far we have concentrated our efforts on Rhododendron clearance at infected sites. We have a bio-security protocol in our woodlands and our staff have been briefed on what action to take. We clear affected sites ourselves but have back-up from Forest Research in the lab and guidance from the Food and Environment Research Agency [FERA]."
For historic and botanic gardens, vigilance is clearly the key. According to RHS plant pathologist Geoff Denton: "There is a concern with the movement of plants. We have had a case in one of our gardens and have had the Plant Health and Seed Inspectorate [PHSI], a division of FERA, inspectors round. We have gone along with their advice to remove and incinerate affected plants. The PHSI also monitors the site."
He adds: "It's not uncommon for pathogens to spread their host range. This new outbreak doesn't change our stance, but we will take on board any new advice from the government and its agencies."
In turn, the RHS advises the public to keep a look out for symptoms of the disease on plant leaves and to have the PHSI inspect possible outbreaks before destroying stock. "The worst thing would be to destroy plants that are suffering from nothing more than water stress," says Denton.
Meanwhile, Royal Botanic Gardens, Kew uses quarantine to isolate and screen seed material collected from areas where Phytophthora is known to occur, even though there is no legal obligation to do so, according to a representative of the garden.
"While seed collectors take care to try and ensure that seeds are free from pests and disease, we feel the additional precaution of germinating and growing on in isolation for a short while is much preferable to risking an outbreak on-site," she adds.
Good hygiene helps to combat disease
"We ask staff to follow good, basic hygiene practice such as keeping work boots at work, sterilising and cleaning tools, isolating and screening woody plant material coming onto site, washing vehicles used to visit other garden locations before returning to site and reminding staff, students and volunteers to report any suspicious symptoms to the plant health officer for identification."
The majority of the 23 infected sites in Scotland have been historic and botanic gardens and at Benmore, the Argyll outstation of the Royal Botanic Garden Edinburgh, staff are being extra vigilant.
Invasive Rhododendron ponticum is rife in adjacent Forestry Commission woodland and is a host of scale insect as well as Phytophthora. As a consequence, staff have to disinfect tools and vehicles when going between different areas.
According to garden supervisor Neil McCheyne: "If you look at the list of vulnerable species, it reads like a stock-take of the garden."
TRACKING THE SPREAD
Phytophthora ramorum is only the latest species from a genus of oomycetes or water moulds responsible for a number of plant pathogens, including P. cinnamomi, which causes dieback in roses and fruit trees and P. infestans, cause of the notorious potato blight.
Mid-1990s First loss of trees in western US, mainly tanoaks (Lithocarpus densiflorus) to P. ramorum. Losses soon run to millions of trees. The pathogen's origin remains unclear, though imported plant material from China is thought most likely.
2001 First appearance of P. ramorum in North American nursery stock confirmed.
2002 First identification of P. ramorum in UK, on a garden centre Viburnum. By the end of the year, the EU imposes control measures based on UK practice that are still in place. While 18 other EU countries have reported the disease, it remains most widespread in the UK.
2003 In Cornwall, first identification of new, related pathogen P. kernoviae (meaning "of Cornwall") in beech and Rhododendron. The Scottish Government Rural Payments and Inspections Directorate begins annual Phytophthora survey of gardens and parks.
2004 First UK survey by the Forestry Commission fails to find any cases of P. ramorum in trees.
2007 First outbreaks of P. ramorum in Scottish and Northern Irish gardens.
January 2009 P. ramorum confirmed on wild bilberry (Vaccinium myrtillus) at a site in Staffordshire.
August 2009 First finding of P. ramorum in Japanese larch, where it appears to be capable of producing spores on the needles.
WHAT TO DO
- Phytophthora ramorum is not easy to spot in the first place. Symptoms differ according to host species and can include leaf blight, lesions and shoot die-back. According to Defra: "Symptoms due to fertiliser scorch, chemical injury, drought stress, freezing damage and sun scorch can also appear similar to P. ramorum infection on a range of hosts."
- If you suspect that a plant has P. ramorum infestation, you are legally required to contact your nearest FERA Plant Health & Seed Inspectorate regional office in England and Wales (see fera.defra.gov.uk for details) or the Scottish Government Rural Payments and Inspections Directorate.
- Cordon off the plant and refrain from handling or moving any parts of it.
- Inspect nearby plants for symptoms, bearing in mind the problem can occur on a wide range of species.
- If the disease is confirmed, you will be issued with a statutory notice detailing eradication and containment measures to be taken. These may include: removal of leaf litter and other plant debris; cutting out infected wood and bark; clearance round affected plants; and even removal and deep burial of soil.