Last year, a Gardeners’ World feature on olive trees said that they were ideal to help create the perfect Mediterranean Garden, but Prof Spence branded the advice “irresponsible” noting that they had been banned from Chelsea Flower Show amid fears Xylella.The disease has ravaged Italy’s olive groves and could decimate UK trees including the oak, elm and sycamore.More than 350 different plants are vulnerable to Xylella, including lavender, rosemary and flowering cherry.But foreign plants can also being other diseases to the British ecosystem. Ash dieback was first officially confirmed in the UK in 2012 when it was found in a consignment of infected trees imported from a nursery in the Netherlands.And fuchsia gall mite, which disfigures plants and is now rife in the South East, has been attributed to an enthusiast illegally importing cuttings from South America. “This is vital if we are going to win the fight to protect our gardens against the growing threat of pests and diseases.”The BBC said that both shows criticised by Prof Spence “Feature advice from horticultural experts and have contributed to the public’s awareness of biosecurity by highlighting the importance of the issue – and will keep audiences updated with any developments which may impact gardeners in the UK.” Defra’s chief plant health officer Professor Nicola Spence Credit:Telegraph A government horticulturalist has criticised BBC gardening shows for encouraging viewers to plant foreign species that risk spreading disease.Defra’s chief plant health officer Prof Nicola Spence branded TV show Gardeners’ World “irresponsible” after a feature on growing olive trees.She said star presenters like Monty Don should not be encouraging gardeners to grow olives because they are a high risk for importing deadly plant disease Xylella fastidiosa.Speaking at a biosecurity event at Tendercare Nurseries in Uxbridge, west London, last week Prof Spence said: “Gardening trends can drive biosecurity behaviour.”I like to watch Gardeners’ World on a Friday night and to listen to Gardeners’ Question Time, but I occasionally have a bit of a rage at them and take to Twitter when I think they’re not providing the best advice, particularly when a listener – somebody in the audience – says ‘I’ve just brought back X from my holidays. What should I do with it?’ Luke Barley, A National Trust ranger at work felling ash trees infected with dieback near Ilam in the Derbyshire Peak District.Credit:Andrew Fox Recently, the Royal Horticultural Society’s director general Sue Biggs warned holidaymakers off bringing foreign plants back to Britain.”For many people, wandering the olive groves of Italy and lavender fields of France are as much a part of the holiday experience as the cities and beaches,” she said.”But we’re asking people to leave these beautiful plants where they are for future visitors to enjoy and not to bring them back home with them. “And I’m screaming ‘destroy’ and they get advised to go and plant it in their garden.”Prof Spence also warned against buying plants from overseas online.She said: “Just because it’s available on Amazon doesn’t necessarily mean it’s actually legal and we are constantly working with Amazon and eBay to track down material that is not compliant.” Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.