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Xylella – the world’s most significant plant threat



February 16, 2016
Leaf scorch from Xylella Photo Pompilid

Oleander leaf scorch from Xylella Photo Pompilid

Plant disease threats just keep getting bigger and nastier and more widespread. If you haven’t heard of Xylella before, you’ll soon wish you still hadn’t. Although burying horticultural heads in sand is what gets us into these biosecurity messes.

Xylella, the symptoms of which go by various names including ‘phony peach disease’ in southern USA and Central America where it’s been around for a while on citrus and coffee plants, Pierce’s disease and also ‘leaf scorch’ of many plants has somehow spread to southern Italy where it has devastated ancient olive groves in Puglia, and southern Italy where its been found on grape vines and lavenders. And it’s now infecting street trees in New Jersey. The host list expands daily, and so far also includes oak, maple, bay, oleander, roses, cistus, prunus, vinca, asparagus, grevillea, westringia, pelargonium, rosemary and hebe.

Yes you read it right, the range of plants that this bacterial disease infects spans a large portion of our ornamental plant database. Scared yet?

It’s most unusual that a bacterial plant disease infects such a wide range of genera. So how is it happening? It turns out that Xylella fastidiosa is able to mutate to adapt to a wide range of climates and hosts. Although it was originally thought that it wouldn’t survive the UK’s colder winters, a new cold-resistant strain in France means that the UK is now on high alert – just in time for the building of Chelsea gardens. Research has already identified two distinct strains, – Xylella fastidiosa subsp. multiplex, Xylella fastidiosa subsp. pauca plus two other Xylella strains that infect coffee.

So what does a Xylella infection look like?

Pierce's Disease (Xylella fastidiosa) on orange tree. Photo Alexander Purcell, University of California, Bugwood.org

Pierce’s Disease (Xylella fastidiosa) on orange tree. Photo Alexander Purcell, University of California, Bugwood.org

Xylella is very hard to diagnose initially, as its symptoms can be similar to those of other diseases and environmental factors. Xylella attacks the xylem transport system in the plant, preventing it from translocating sugars and water, leading to wilt, leaf scorch, widespread canopy dieback and eventual plant death.

In the UK all professional ‘plant operators’ will now need a ‘plant passport’ to bring in any known host plant from the EU into the UK. If an infection is detected in the UK, strict new biosecurity measures will kick in, including removal of every known host plant within in 100m radius (which could be most of a garden) and even stricter controls on plant movements in the surrounding 10km zone. Which could potentially destroy a local nursery’s business and prevent landscaping works on a massive scale.

In Australia, the most likely point of incursion will come from Pacific Islands, some of which already have Xylella fastidiosa and its common vector, the glassy-winged sharpshooter, a sap-sucking leaf hopper. If the bacterium arrives in Australia, several other leaf hoppers, including cicadas, already present here are also potential carriers.

EU database of known host plants for Xylella HERE

See UK DEFRA Xylella warning and guidance document HERE

See Australia Xylella warnings and alert HERE

(Many thanks to Sandra Simpson of Sandra’s Garden for her blog post spreading the Xylella alert)

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