Gardeners are being urged to report sightings of spittle on their plants in a bid to stop bugs spreading a deadly disease through the UK.

The Royal Horticultural Society (RHS) issued the plea as it described the Xylella fastidiosa bacteria, which prevents water travelling from roots to leaves, as its “number one concern”

While the disease has not yet been spotted in the UK, it has already taken hold in France, Spain and Italy, where it has killed millions of olive trees.

The bacteria is spread by a small insect known as a spittlebg or froghopper which moves from plant to plant to feed on sap.

They are named after the small white blobs of spittle left by their nymphs on leaves and branches.

The most common species is the meadow spittlebug, which is around five millimetres long and can vary in colour and pattern from black to brown.

Volunteers can report sightings of spittle on the Biological Records Centre’s iRecord website.

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The RHS, together with Forest Research and the University of Sussex, will use data about the bugs to track the threat of Xylella in the UK.

If Xylella is discovered, all host plants within 100 metres would need to be destroyed.

There would also be an immediate restriction of movement for some plants within a five kilometre radius, the RHS said.

More than 500 plants are at risk, including lavender, oleander, rosemary and flowering cherry.

Gerard Clover, head of plant health at the RHS, said: “Xylella remains our number one concern but is not an issue bound by the garden fence.

“Understanding how and where the disease’s primary vectors move is fundamental to understanding how we can stop the devastation of our gardens and environment should it arrive.”

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Alan Stewart, of the University of Sussex, said: “Records of spittle submitted by the public will help us to build up a picture of where spittlebugs are found, what plants they feed on and how much they move around.

“This information will be essential for deciding how best to respond should Xylella arrive in the UK.”