The Xylella fastidiosa bacteria is native to America but has already taken hold in France, Spain including the Balearics, and Italy, where it has killed millions of olive trees. Although it has not yet been detected in the UK, the RHS has serious concerns about the risk of introduction via infected, imported host plants. The bacterium is transmitted between plants via insects which feed on plant sap (such as the meadow froghopper) and which produce spittle to protect their nymphs.
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​Xylella infects a wide range of garden plants. Symptoms of infection include leaf scorch, wilt, dieback and eventual death. RHS has some photographs to help identify infection.  Meanwhile, it makes sense not to import any plants ourselves.