Xylella control research proposed

Bacteriophages could be worth researching to find a solution to xylella fastidiosa, say horticulturists.

Flowering Plants’ Francis Richardson has put forward the idea of using the long-established science to tackle the new disease, which is threatening the European plant trade and for which there are no authorised controls and has led to Defra secretary of state Michael Gove threatening to ban imports of hosts including rosemary, lavender, nerium oleander, polygala and olves.

Dr Tim Pettitt, senior research scientist at Charles Darwin Labs, University of Worcester, said: "On the face of it using bacteriophages to control Xylella sounds like an elegant solution and there have been claims to control success against Pierce’s Disease (Xylella fastidiosa subsp. fastidiosa) in grapevines (Das et al., 2015) although only under very controlled (‘artificial’) experimental conditions.  The use of phages for control of bacteria in plants has quite a few practical limitations – phages are not very persistent and break down fast when exposed to natural daylight (for instance on leaf surfaces). 

"They need to be present in high concentrations to be effective, so timing and the nature of application would be absolutely crucial. And their specificity means that appropriate strains of phage need to be deployed to achieve any hope of control. This means that the composition of the population(s) of target bacterial pathogen need to be closely monitored to make sure that the phage strains used remain the most effective.  The positive side to this specificity means that there’s a low risk of unintentionally hitting non-target ‘good guys’. 

"I think very careful consideration of the strategy needs to be taken and a willingness to adopt ‘unconventional’ methods as opposed to just spraying, is needed for the phage approach to stand any chance of success.  There would also need to be a great deal of research carried out on optimising the production, stability/durability, formulation, application procedures and efficacy before the approach could be used with confidence."

Richardson said Pettitt's insight is valuable, adding: "I fear that Xylella will be no respecter of the time scales required by regulators."

Rationale Biopesticide Strategists' Dr Roma Gwynn said: "From a global consideration: yes, there are bacteriophages isolated against Xylella fastidiosa and they have shown in research to have some efficacy potential but most research to develop a product has been for this disease in grapes."

She said bacteriophages tend to be crop and/or plant species specific so a UK specific solution would need to be found and to develop such a technology from research into a product will take commercial involvement. The active substance and product would require registration as a Plant Protection Product and for commercial development it would be a necessary to have a partner or lead. 

Former HDC chairman Neil Bragg said: "I can see the logic. It's just whether or not you can build up enough numbers and get it regulated under biocides directives."

Nursery consultant John Adlam said: "I'm open to the possibility. This concept is something that has been known about but certainly not developed because there has not been the need for it."

Adlam added that copper-based products could control xylella and he has discussed with CRD and chief plant health officer Nicola Spence having them authorised for use as part of a plan to combat the disease in case it reaches the UK.

Francis Richardson's route map:

1.  There seem to be many forms of Bacteriophages. 

2.  They have been seen to arrive spontaneously, for example in two
nurseries in the UK where Fusarium has disappeared from Hebes, and on one in
Holland where Erwinia and Pectobacterium disappeared from Zantedeschia.

3.  Because they are so very small (Large quotes Japanese work back to 1914,
indicating diameters between 25 and 64 microns) they would appear to be able
to live wherever pathogenic bacteria live within the systems of plants.
            Edward C. Large  "The advance of the fungi."  Hollen Street
Press 1940, then Jonathan Cape 1958.  Pages 425 and 426.

4.  Work in Georgia showed that they are often present in
surprisingly large variety. Three researchers' papers showing how
that and other work in Bulgaria was taken up by the USDA from 2007 onwards.

5.  The method of the Manchester system does not introduce any life forms;
it simply encourages beneficial ones to multiply.  They seem to do so
provided that the system is appropriately balanced.

6.  As with entomopathogenic Nematodes, you do have to develop some skill in
putting natural organisms to work for you.  

7.  In anticipation of an EU-organized conference to take place in November
work is on-going at Bari to identify life forms found in association with
Xylella Fastidiosa in the vascular system of Olives.  See: https://www.efsa.europa.eu/en/events/event/171113

The conference on European research into Xylella fastidiosa is to be held in Palma de Mallorca, Spain, from 13-15 November 2017. The conference is being organised jointly by EFSA, the University of the Balearic Islands, the Euphresconetwork for phytosanitary research coordination and funding, the EU Horizon 2020 projects POnTE and XF-ACTORS, and the European Commission’s Directorate-Generalfor Research and Innovation (DG RTD).

The event will provide a platform for in-depth discussion on the results of research into X. fastidiosa and its vectors, in support of on-going efforts to control the European outbreaks. As well as speakers and participants from Europe, the conference will be attended by scientific experts from other parts of the world – such as Brazil and the United States – where X. fastidiosa has been present for many years.

Industry views:

Nursery consultant John Adlam said xylella could devastate UK plant production and sales and that the UK industry had suffered after chalara, sudden oak death and Asian longhorn beetle outbreaks: "I would say the UK growers are not over-reacting but are in fact taking a very sensible precautionary view, understanding full well the disaster that could occur if this disease is found in the UK."

Hayloft Plants' Derek Jarman said: "It's common sense. Don't import a problem." he said importing plants from from the host areas was bad for gardeners and growers.

He said those that had decided to stop importing from wider areas or whole genuses were making a "commercial decision that is right - no-one wants to bring in trouble".

But he said a campaign to raise public awareness, which he said was low, could be seen as a "negative" and lead gardeners to not buying plants: "It's up to the industry to take sensible precautions. People might be put off buying plants in general so its best to shut up and get on with it."

Buckingham Nurseries' Chris Day said the garden centre tends to use third party UK-based growers that import and don't import direct themselves, mainly because minimum order values caused by carriage costs are too high.

The nursery imports from Italy and Holland at the beginning of spring when stock is acclimatised to be brought on to the same level of the UK summer. But they do not import later in the season because of the risk of plants dying.

He said there was still demand for olives in all formats although supermarkets stocking olives had "diluted" sales in garden centres.

Day said he sources rosemary and lavender from UK growers when possible though some named varieties do come from Holland. He said Buckingham does not deal in large Italian-grown one litre stock.

He said more awareness is needed to get the message out to consumers about the potential dangers of xylella, adding: "We're doing our bit being careful where we get our stock from. But a better awareness campaign in the consumer market might be good, especially in spring when people will be buying Italian and Spanish stock, which is more susceptible."

Palmstead Nurseries Nick Coslett has decided not to import high risk specimens and will now seek to increase production of plants such as rosemary and lavender, and find alternatives to specimens such as olives.

UK nurseries such as Boningale, Johnsons of Whixley and Majestic Trees have been vocal on how they are taking a stance to stop the disease entering the UK.

Coslett added: "We imported rosemary and lavender at those times of the year when our production was on the low side. We will increase our production to fill that sort of gap.

"We have to act responsibly as a grower, importer and supplier."

Palmstead is also starting to grow more phormium again.

He said he will stop importing any olives and have to hope designers understand when current quarantined stocks are sold they can't get any more in.

He suggested willowleaf pear, cornus, arbutus or pines as alternatives.

Coslett said biosecurity in some of the Mediterranean countries hit by Xylella is "tricky" and "we're not able to rely on these nation states' own plant health systems - we can't rely on them to protect the UK".

He said Defra don't want to "rock the Brexit boat" too much though believes Defra secretary of state Michael Gove wants more restrictions in place. Some continental growers say EU rules are too draconian and are driven by the French to protect their wine market.

There are no controls and no way of stopping Xylella migrating through southern Europe. The EC extended the 300-plant host list this month.

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