Xylella fastidiosa has wreaked havoc in the US and Europe and could dwarf the impact of ash dieback in the UK

It has caused severe damage to plants and trees in the US and southern Europe and now there are fears it is heading this way. With experts warning that it could make the devastating ash dieback disease seem like “a walk in the park”, the UK is on red alert for signs that Xylella fastidiosa has entered the country.

First confirmed in Europe three years ago when it ran rampant across olive plantations in southern Italy, a subspecies of Xylella has since been detected in southern France, where it has destroyed vines and lavender plants, and in Corsica. Xylella fastidiosa has also been found in both South and North America where it is commonly referred to as “phony peach disease” and where it has caused severe damage to citrus and coffee plantations. In New Jersey it has attacked more than a third of the state’s urban trees.

According to guidance issued by the Department for Environment, Food and Rural Affairs (Defra), Xylella has already infected oak and maple trees, hebe (an evergreen shrub), lavender and rosemary. It results in multiple symptoms, including wilting, diebacks, stunted growth and leaf scorches.

Raoul Curtis-Machin, head of horticulture at the Horticultural Trades Association, the body representing the UK’s growers and landscapers, said it had initially been thought that Xylella could not survive in the UK because of the climate.

“But last autumn the Animal and Plant Agency discovered a new strain, which is cold-hardy, in Corsica and France,” Curtis-Machin said. “It’s getting very close to Britain. What has alarmed us is that it’s quite difficult to spot and it affects a massive list of different host plants. The list has got so big that the EU has stopped publishing a [hardcopy] list and just publishes it online.”

Unlike many plant diseases, Xylella has the potential to affect multiple species. Curtis-Machin drew comparisons with the way flu viruses affecting humans can mutate.

“Like flus and colds, it’s constantly changing. There are a lot of different Xylellas out there and this new one would make Chalara fraxinea [ash dieback] look like a walk in the park. It affects trees, shrubs, herbaceous plants like lavender, rosemary, bay: all the classic plants you see in pots outside houses.”

Horticultural experts have been lobbying hard for the EU to extend plant passport regulations, which currently apply only to wholesale importers. “But that’s only one chunk of a huge trade,” Curtis-Machin said. “There are a lot of garden designers going over to Italy and importing big olive trees. God knows what’s on them. You may not see the symptoms and a year later it could be hopping out to the plane trees, and before you know it you’ve got the trees in the avenues of The Mall dying off.”

According to the Forestry Commission, “Xylella fastidiosa affects its host plants by invading their water-conducting systems, moving both upstream and downstream. In so doing, it restricts or blocks the movement of water and nutrients through the plant, with serious consequences, including death, for some host plants.”

An outbreak in the UK would result in the introduction of stringent emergency control measures. All known host plants within 100 metres of the outbreak would be destroyed. Sweeping restrictions on the movement of plants within a buffer radius of 10km of the outbreak would be imposed for five years.

This could have potentially devastating consequences for urban landscape gardeners. For example, an outbreak on a landscape site in Canary Wharf, east London, could result in the near shutdown of landscaping work in the capital.

The HTA says it alerted the government to the risk of importing ash dieback back in 2009. To combat another disease taking hold, the association is trying to convince Defra and other agencies of the need to develop a secure market for UK-grown trees.

But, despite pledges to improve the amount of woodland cover and tree planting in the UK, the amount of new planting in England has dropped from 3,300 hectares in 2013-14 to 2,400 hectares in 2014-15. Experts believe the yet-to-be published figure for 2015-16 could be as low as 1,600 hectares.

An increasing reliance on imports could see more growers turn their backs on a £300m industry, something that would have consequences for the UK’s biosecurity. Mature trees used in landscaping take more than 15 years to grow. But fewer growers will commit to such long-term goals if they have doubts about the government’s commitment to domestic tree growing. Any decline in the UK’s tree population could affect how the country combats threats posed by climate change.

“Trees have a stabilising effect on the climate,” Curtis-Machin said. “Trees absorb air pollution. You’ve got horrific levels of air pollution outside primary schools right now in a lot of our urban areas. As we’ve seen with the recent flooding, without trees water catchment is shot to pieces. We know that trees stabilise banks, so without them you end up with a lot more landslips and you get a lot more road closures.”