Xylella: further work needed on role of grapevine

Grapevine cannot be ruled out as a potential host plant for the strain of Xylella fastidiosa that is attacking olive trees in southern Italy, EFSA’s plant health specialists have concluded.

EFSA’s conclusion was delivered as part of its evaluation of recent studies and experiments carried out in Apulia, where the bacterium X. fastidiosa strain CoDiRO is known to have been present since October 2013.

Stephan Winter, who led the working group that evaluated the information, said: “The results from the studies conducted so far are very promising and provide a good basis for further confirmatory work that could rule out grapevine as a host for this particular strain of the bacterium.”

“However, our assessment suggests that although grapevine is not a principal host of the CoDiRO strain – and plants apparently do not succumb either in the field or in the laboratory – there is currently no unequivocal evidence that grapevine could not act as a source of infection.”

The information examined by EFSA was made up of: surveys from vineyards located in the heart of the zone where X. fastidiosa strain CoDiRO is established; results of inoculation experiments to infect grapevine with X. fastidiosa CoDiRO; and results of transmission experiments using insects as vectors of X. fastidiosa CoDiRO.

The field surveys yielded no evidence of infected grapevine, and none of the grapevine plants used in the experiments became infected either through inoculation or via contact with insects. However, the working group identified uncertainties which meant that it was unable to exclude the possibility that grapevine is a host plant of CoDiRO.  

The experts noted that, although the field surveys were negative, no information is available on infective vector populations present in the vineyards.

On the inoculation experiments they raised question marks over the limited number of vines used, the stringency of the inoculation procedure and the use of only one vine variety.

For the vector transmission experiments, uncertainties centred on how active the insects were and how much bacteria they may have transferred to the test plants.

Dr Winter added: “Diseases caused by Xylella fastidiosa are the result of highly complex interactions between pathogen, vectors and host plants. There are so many unknowns – such as insect behaviour and environmental conditions – that experiments of this kind are fraught with difficulty and uncertainty.”

Background

Species of Vitis are included on the list of plants that have been subject to control measures since the outbreak of X. fastidiosa in Apulia. The dossier examined by EFSA was submitted to the European Commission by the Italian authorities in support of a request to have Vitis removed from the list.

The new evaluation follows an EFSA Scientific Opinion published in September 2015 which concluded that hot water treatment is an effective safeguard against all strains of X. fastidiosa in grapevine planting material.

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