Xylella: is there scope for a cure?

A scientific conference on European research into Xylella fastidiosa held in Palma de Mallorca, Spain will showcase the worldwide work being done to tackle the disease.

The conference, on 13-15 November 2017, is being organised jointly by EFSA, the University of the Balearic Islands, the Euphresco network for phytosanitary research co-ordination and funding, the EU Horizon 2020 projects POnTE and XF-ACTORS, and the European Commission’s Directorate-General for Research & Innovation.

The event will provide a platform for in-depth discussion on the results of research into X. fastidiosa and its vectors, in support of ongoing 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 USA, where X. fastidiosa has been present for many years.

Flowering Plants’ Francis Richardson says he is particularly hopeful about research on the endophytic bacterial community of healthy and Xylella-infected olive sapwood by Franco Nigro of the Università degli Studi di Bari "Aldo Moro" - Dip. Scienze del Suolo, della Pianta e degli Alimenti, Bari, Italy, who is speaking on 13 November.

Event speakers are mainly from countries hit by the disease — Italy, Spain and France. There are also speakers from the USA, Brazil, Costa Rica and Belgium plus three from Britain:

  • Potential insect vectors of X. fastidiosa in the United Kingdom by Chris Malumphy of Fera Science
  • Developing surveillance strategies for X. fastidiosa in the Mediterranean region by Alexander Mastin of University of Salford
  • Modelling the spread and control of X. fastidiosa in the early stages of invasion in Apulia by Steven White of NERC Centre for Ecology & Hydrology.

White says his scientific model could help predict the spread of Xylella. The model can qualitatively and quantitatively predict how the bacterial pathogen may spread as well as offer guidance on how buffer zones should be arranged to protect uninfected olive trees.

The research, published in the journal Biological Invasions, highlights how X. fastidiosa is influenced by a range of insects, including spittlebugs, and the rate to which these vectors contribute to the potential spread of the disease across Europe and beyond.

The study modelled control zones in Apulia, Italy, and found that increasing buffer widths decreased infection risk beyond the control zone but may not stop the spread completely. This was due to the ability of the disease-spreading insects to transport themselves between sites.

White says: "This work provides policymakers with scientific information about how the disease might progress and how the control strategies imposed by the EU may reduce the risk of spread to the rest of Italy and beyond."

Mastin's area is surveillance and epidemiology - simulation models to help inform and improve early detection surveillance strategies for Xylella fastidiosa, with a particular focus on the situation in Italy.

There is no-one involved looking at cures or treatments for xylella in the UK. AHDB's Wayne Brough says AHDB is undertaking a review of bacterial diseases (Steve Roberts, Plant Health Solutions) before commissioning work, and the HNS panel has been discussing the subject of Xylella and reviewing what needs to be done specifically for the sector. Several AHDB staff hope to attend the Spanish conference.

Dire consequences

Speaking at Majestic Trees' 15th anniversary landscape industry biosecurity symposium, chief plant health officer Nicola Spence discussed the dire consequences of the disease coming into the UK and stressed that it has no known cure. 

Areas hit by Xylella include Corsica, parts of Provence, Alicante, Liguria, Monaco, Saxony, Thuringia and Apulia. Spence said the most worrying plants hit are olives, lavender, rosemary, almond, polygala, oleander and coffee.

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 olives.

Dr Tim Pettitt, senior research scientist at Charles Darwin Labs, University of Worcester, says: "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 [X. 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 says Pettitt's insight is valuable, but adds: "I fear that Xylella will be no respecter of the timescales required by regulators."

Potentially effective

Rationale Biopesticide Strategists' Dr Roma Gwynn says: "From a global consideration, yes, there are bacteriophages isolated against X. 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."

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, she adds. The active substance and product would require registration as a plant-protection product and for commercial development it would be necessary to have a partner or lead. 

Former HDC chairman Neil Bragg says: "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 adds: "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." He points out that copper-based products could control Xylella and he has discussed with the Chemicals Regulation Division 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.

Adlam warns that Xylella could devastate UK plant production and sales, and says the UK industry has 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."

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 show 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. Richardson is waiting in anticipation of the EU-organised conference to take place in the Balearics on 13-15 November, especially ongoing work in Bari to identify life forms found in association with X. Fastidiosa in the vascular system of Olives. See: https://www.efsa.europa.eu/en/events/event/171113.

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