Polygala is particularly under threat after Defra said more than 60% of positive diagnoses in outbreaks in France have been on the shrub.
Defra said the plant is a "very high risk host" and that there are "large volumes of trade that sometimes moves through the distribution chain to retail very quickly".
The plant has been hit by X.Fastidiosa in Mallorca, X.Pauca in Italy and Ibiza and X. Multiplex in Mallorca and France.
The polygala is one of the most at risk plants and is among seven Defra has identified as "high risk". The others are olive, rosemary, lavender, prunus and Nerium oleander.
Alton Garden Centre director Andy Bunker, who is also a Tillington group buyer, said: "I will probably stop selling polygala." Bunker sells up to £5,000 worth of polygala a year, which suggests the UK trade in the plant is worth hundreds of thousands of pounds.
Alton and Hillview were among 16 garden centres companies who signed an HTA letter this summer saying they would not buy plants from areas where xylella is known to exist.
Hillview Group chief executive officer Boyd Douglas-Davies said: "We won’t be purchasing any of the six key host plants. In addition we won’t be buying any of the plants on the larger host list from the infected countries. We have decided that the regions in these countries are just too small and the risk of it travelling is greatly increased within any of the countries. We are currently checking with all plant suppliers on their policies and procedures."
Several wholesale nurseries including Bransfords, Johnsons of Whixley, Coles, Golden Grove, Oakover and Lovania have also signed an HTA statement to "not knowingly purchase any host plants originating from regions where the disease Xylella is known to exist."
Areas involved include Corsica, parts of Provence, Alicante, Liguria, Monaco, Saxony, Thuringia and Apulia.
Members of the HTA tree and hedging group and several retailers signed similar statements.
But Johnsons of Whixley, Glendoick and Boningale have since spoken out against perceived lack of stronger statements against xylella and said they will not import from whole countries where regions are affected.
Boningale Nurseries chairman Tim Edwards says consequences could be that buyers would, for example, need to accept northern European lavenders in early season rather than southern European plants.
An outbreak of X. fastidiosa on a nursery could result in the destruction of all host plants (300 species) within 100 m, and implementation of a 10 km zone banning all host plant movements for five years.
Nursery consultant John Adlam said: "If a nursery had an outbreak and that 10km radius comes into play, my fear is there could be litigation for loss if trading. This brings even more of a pressure on people to be very careful about where they buy from. So it's not just the biosecurity element, it's the litigation element as well."
He adds that there is concern about landscapers and building developers who buy plants direct and retailers who buy from traders.
Four Oaks trade show (September 5-6 Lower Withington, Cheshire) sent an email to its 420 exhibitors saying all xylella host plants are subject to plant passporting. More than 100 grower exhibitors are from overseas.
APHA inspectors will be present at the show during build up to inspect all incoming nursery stock.
Bunker said plant passporting generally can be lax and xylella will lead to an all-round tightening of checks.
He added: "I’m not going to buy plants from that part of Italy [where xylella has been identified] – that’s common sense. It’s quite concerning but there’s got to be a balance with what’s going on and common sense. If we’re not careful we’ll stop all imports from Italy."
Bunker was in Italy recently and said importers were being vigilant about Xylella.
Plants for Europe owner Graham Spencer said: "Any plant that is a Xylella host and relies on an international (not just European) supply chain is at risk of shortage. I wonder how closely buyers will stick to their Xylella buying pledges if sourcing becomes difficult for some important item. But I also see evidence that growers and propagators are anticipating problems by establishing production in secure locations in Xylella-free areas."
Gardening Express owner Chris Bonnett predicts shortages of plants from southern Europe, mainly because of high temperatures there, but also as areas are shut off from trading because of Xylella. he says online sellers are more flexible than bricks and mortar retailers if Xylella and its exclusion zones hit the UK, because online sellers can easily move away from non-movement zones. Bonnett says polygalla is still being offered but as a prime Xylella host, he expects import to the UK will dwindle. But he says palms from non-affected areas will still be available, though potentially olives could be in short supply.
Coolings managing director Gary Carvosso said Coolings was now unlikely to stock xylella host plants polygala, and lavender and rosemary that are imported from southern Europe in January. He added that olives would potentially be de-listed too to avoid the chance of bringing the plant disease in from affected areas on the continent.
Squire’s Garden centres managing director Martin Breddy said he was "delighted" the HTA and industry generally has taken Xylella "extremely seriously". Squire’s also signed the HTA Xylella statement and Breddy added: "Maybe I’m an optimist but I hope the measures taken will enable us to find sources that are clean. I know some people are very, very converned about it and feel it’s just a matter if time [before Xylella reaches Britain]."
He said there had been a "feverish reaction" by some "but I think measures taken are really proportionate to the threat".
Breddy said the question needed to be asked about how catastrophic xylella had been to internal trade in Italy, Spain and France. In Puglia, declaration of a 'natural disaster' after drought and Xylella led to a 50% reduction in olive oil production, has forced authorities to reduce agricultural social security contributions and suspend/extend agricultural tax deadlines as well as give €5m to extend mortgages.
Defra has produced the following guidance for those importing plants:
- Sourcing plant material: source plant material from known suppliers and visit suppliers to view their processes, procedures, biosecurity arrangements, any assurance schemes they may belong to and the plants they grow.
- Check the plant health status of the supplier/country.
- Ensure that imported plants both originate from, and are sourced from, disease-free areas.
- All professional operators are obliged to ensure that potential host plants of quarantine pests and diseases imported from the EU are accompanied by a plant passport confirming they have been sourced from disease-free areas/sites.
- Nursery bio-security measures: Ensure that plant passports arriving with plants are correct and keep the plant passport to aid trace-back if necessary.
- Isolate or quarantine new batches of plants and monitor them for signs of the disease and place imported hosts of notifiable pests and diseases in a quarantine area a good distance away from other host plants or under physical protection. With Xylella all host material within 100 m will need to be destroyed.
- Label and keep records of the identity of all received batches of plants, including the source and date.
- Maintain good records of pesticide treatments.
- Destroy old or unusable plants, and maintain records of the method used.
- Comply with the UK national requirements to notify the UK Plant Health Service about certain species of plants under the EU Plant and Tree notification scheme.
- Suspected outbreaks of a quarantine organism or non-native plant pests must be reported to the relevant Plant Health Service authority.