The EU's ban on importing Acer from China is the latest move to contain the spread of citrus longhorn beetle (Anoplophora chinensis). But this is only the most recent fight back against exotic nursery pests and as such part of a war that can never be won outright.
RHS principal entomologist Andrew Halstead says: "I think it's definitely a growing problem. There are a lot more new pests and diseases turning up than there were 20 years ago. It's partly down to a changing climate - we have longer summers and milder winters than in the past.
"There's been a great deal of growth in international trade. It's often cheaper to import plants from far and wide than to grow them in Europe, so it's probably a fact of life now. We have international trade agreements so we can't just pull up the drawbridge - and growers are powerless against the climate."
Controlling the spread of novel pests is the work of the Plant Health & Seeds Inspectorate of the Food & Environment Research Agency (FERA), part of Defra. But Halstead points out that you first have to know what you are looking for. "We only found out about Phytophthora kernoviae when inspectors went looking for P. ramorum," he reveals.
"Here at Wisley we often find out about new pests, such as horse chestnut leaf miner or Fuchsia gall mite, when RHS members send them to our advisory service - and there are plenty more things in South America and Asia that could still come our way." The RHS quarantines new plants before they are planted in its glasshouses. "But you cannot do that for every plant," says Halstead.
FERA plant health policy officer Richard McIntosh says the agency has to prioritise where to direct its efforts. "We have a regime in place to inspect high-risk material - we call it a risk-targeting approach," he explains. "There are also pests that are 'of concern', where we inspect a proportion of plants. If we hear of repeated problems on uncontrolled material, we bring in formal checks there too. If you are inspecting a plant 100 per cent without finding the pest, you can discuss whether that's still appropriate."
The agency also has regular meetings with EU counterparts to pass on information on possible new threats. The Standing Committee on Plant Health is responsible for plant health directives and other instruments providing a common approach to pests across the EU, with citrus longhorn beetle currently classed as "highest risk".
"We can't do anything about endemic pests," says McIntosh. "But there are many thousands of potentially harmful organisms that are not yet here - of which longhorn is one of the most pressing."
FERA also has to keep an eye out for pests from other sources, he adds. "It could be any number of things that alert us to a problem. A pest could be found on non-controlled goods. Inspectors of plant passporting nurseries will look for anything unusual on other plant lines, or they could be picked up at non-passported nurseries, wholesalers or retailers. We encourage the general public to report sightings too."
Nursery consultant John Adlam says growers too cannot drop their guard. "The current level of vigilance with imported material should be maintained - it should already be at a high degree. Many common pests have come in and are still coming in from EU member states."
But he defends current control mechanisms. "These days our communications machinery is quicker and more effective, so the situation is not necessarily worse. The average grower will tell you that the current protocols are robust to the point of being arduous, though the current financial climate might mean there are fewer inspections.
"Officers can't inspect everything that comes in. But from the grower's point of view, it's better for destructive sampling to be carried out at the port because at that stage he hasn't already paid for the plants and the pest hasn't had a chance to escape onto other plants on his nursery."
But control of imported plants is not enough to stop the problem, Adlam warns. "Like Dutch elm disease, citrus longhorn beetle didn't just come in on plant material but also on palette wood. The vigilance of the grower is not the only necessary form of defence," he adds.
Nor does he see the problem going away any time soon. "British growers have always been opportunists when it comes to bringing in plants from other countries. People were importing plants from New Zealand 25 years ago. But globalisation increases the risk. We're bringing in cuttings from South and Central America, where it's cheaper to produce them - it's a good way to go in some ways, using the advantages that different areas have in producing different crops, though I would also like to think the UK industry could expand at their expense.
"Indeed, many growers will tell you it's less hassle to source from within the UK, even if it isn't cheaper. P. ramorum, for example, has actually been good for the British Rhododendron industry."
Nick Hourhan, owner of Spring Reach Nursery, a retail nursery in Surrey, backs this up. "We mostly buy English-grown stock," he says. "But a lot of our customers are having problems with Rosemary beetle, which will strip off the foliage, and it is on the increase. Things that were notifiable five or ten years ago now no longer are, like mealy bug on Phormium.
"A lot of it has come about with the growth in trade. Internationally, you have huge micro-propagation of things like Heuchera now, and climate change is playing a part too. There's a big question on how to police it, especially when there's less money there. It's a potential time bomb."
Kernock Park Plants managing director Bruce Harnett adds: "There's always going to be a compromise between bureaucracy and efficiency. It would be a concern if the checks aren't effective or the rules are relaxed. But it's a minefield. Continental growers import plants, grow them on, then export them here - that will always be an issue. They have to be picked up when they first come into Europe, which is not always easy. The more you import and export, the more chance you have of transfer."
He says his Cornwall-based nursery is "hot on checks", in addition to those made by FERA. "It's in our interest and so far this year we've not found anything. But then we work with suppliers who themselves have good controls, so it shouldn't happen."
For further information on specific pests and diseases, see www.hortweek.com/resources/PestsAndDiseases/
Top ten for trouble
The risk of exotic pests to the hardy nursery stock industry changes constantly, but right now the ten below are among those causing the most concern. FERA continually assesses its response and welcomes comments from concerned parties. These can be emailed to firstname.lastname@example.org.
Andromeda lace bug (Stephanitis takeyai) A small flying insect native to Japan, this is a mainly a problem on Pieris, where it causes leaves to mottle and drop. Now considered established in the UK. No effective treatments are available.
Box tree caterpillar (Diaphania perspectalis) A far-eastern moth larva up to 4cm long, first found in Britain in 2008. Causes leaf loss and die-back, compounding existing problems with box blight (Cylindrocladium buxicola) on Buxus.
Citrus/Asian longhorn beetle (Anoplophora chinensis, A. glabripennis) Two far-eastern boring pests affecting a wide range of trees and shrubs. Infestations of A. chinensis in Belgium and Holland led to emergency control measures earlier this year. According to FERA: "In the UK, long-term establishment outdoors is unlikely but significant damage is still possible."
Fuchsia gall mite (Aculops fuchsiae) Too small to be seen with the naked eye, this notifiable pest has been reported in several southern England sites since 2007. Causes galling (distortion) of leaves on all but the most resistant Fuchsia. FERA describes it as "difficult to eradicate".
Phytophthora spp. The oomycete (water mould) P. ramorum is already an established pest on a range of woody plants on the wetter west side of the UK, where the newly-discovered P. kernoviae is also prevalent. Most recently a third pathogen, P. lateralis, has been found on Chamaecyparis.
Pine needle rust (Coleosporium asterum) A fungus that damages needles on young trees but requires flowers of the daisy family to complete its life cycle. Not yet in the UK, but FERA says: "Could enter on cut flowers or plants from North America, Asia or Africa."
San Jose scale (Quadraspidiotus perniciosus) A sucking insect originally from east Asia, this pest of fruit trees and bushes has been a major problem for more than a century in the USA and is also established in much of continental Europe, though so far not in Britain.
White peach scale (Pseudaulacaspis pentagona) A polyphagous (of many hosts) insect pest, capable of killing some plants. Established in many parts of the world and detected in several plant consignments to the UK over the past 15 years.
Wisteria scale (Eulecanium excrescens) Sap-sucking 1cm-long insects that can cause lethal damage to Wisteria and also threaten Prunus. Thought to have been brought in on plants from Asia, it has been a growing problem since establishing in the London area in 2001.