Transport of infected plant material is the main way by which sharka disease spreads over long distances. More locally, PPV is transmitted by aphid vectors in a non-persistent manner. It is a brief event (time-range of a few minutes) resulting from the probing behaviour shared by all aphid species. As for other non-persistently transmitted viruses, over twenty species of aphids, including many ones which are not hosted by Prunus, can act as vectors of the virus. It should be stressed that in this type of transmission the infectivity is lost through the aphid’s moults. Thus, the winged aphids are non-viruliferous when they take off for their first long distance emigration flight even if they where on an infected plant (and unless they have probed on their host before departing, which is not reported as a normal behaviour). This has two consequences on the virus dissemination: (i) the aphid species hosted by Prunus trees are not necessarily the main vectors, and (ii) the dissemination is mainly due to aphid visitors, i.e. aphids that perform short range flights in their search for a suitable host plant. Consequently, long distance (more than 1000 meters away from the source of inoculum) dissemination by aphids is thought to be very unlikely. Due to these aphid transmission characteristics, it can be assumed that introduction of the disease into new territories away from existing infection loci is the result of the human trading activities, while local spreading can be due to both aphid activity and to agricultural practices. After its introduction, a key parameter for the spread of the virus is the quality of the inoculum sources available for aphids, the prevalence of the disease and the vector activity in a given ecological area.
• Identification of the pathogen diversity and spreading driving forces
After PPV introduction in an orchard, the installation of sharka disease varies depending on the identity of the PPV strain and the Prunus species involved. Epidemic properties at a larger scale are still under speculation. Moreover, the recent discovery of highly divergent PPV isolates in neighbouring EU countries (Moldavia, Turkey, Serbia) indicates that much remains to be unravelled about PPV distribution and diversity. This is a key issue in the elaboration of a PPV risk management system. In this context, the project will focus on the diversity of the virus and on the vector-virus-host interaction properties:
- characterisation of the emergence of new PPV variants. This will be achieved through the development of high throughput typing tools for PPV isolates and the understanding of the occurrence and origin of new PPV variants. Models of PPV emergence will be proposed and discussed.
- characterisation of PPV outbreak mechanisms through the mapping of PPV diversity and characterisation of epidemic properties. Models and different scenarios of PPV outbreak will be analysed and tested.
• Identification of genes and loci controlling resistance to the sharka disease
Breeding for resistance to PPV encounters the usual problems of breeding perennial plants together with the difficult procedure of screening for PPV resistance that hinder the programmes. In SharCo, we will achieve the development of molecular tools that will be used to accelerate the selection of seedling progenies in the first steps of breeding programmes, on seedlings. In apricots, implementation of marker assisted selection for resistance to PPV will be achieved in the EU-member states, by transferring tools and knowledge to breeding stations in Romania, Bulgaria, Serbia and Turkey.
However, although natural sources of resistance have been identified in some Prunus species, they are limited in number. In order to diversify the mechanisms of resistance to sharka disease, we will test new biotechnological strategies during the SharCo project. We will achieve the identification of Prunus plant proteins necessary to the virus and develop strategies (natural mutants, RNAi constructs) which interfere with PPV infection. Still-unknown host proteins restricting viral infection will be identified in the model plants Arabidopsis thaliana or Nicotiana benthamiana and will be transferred to Prunus to assess their impact of resistance. Complementary strategies will be evaluated, from the expression of recombinant antibodies or virus-derived constructs to the interference in glycosylation or phosphorylation of PPV proteins. The results expected from this part of SharCo will thus be the development of traditional and innovative approaches for a durable resistance to sharka disease.
• Tools and strategies for outbreak containment
The spread of the non-persistently transmitted viruses is not affected by the use of pesticide treatments against aphids. In some instances, insecticides may increase, rather than suppress the spread of virus transmission by destructing the predators and parasitoids targeting aphids or by causing increased vector activity. Another control measure, the biological control of the vector, is also not an effective strategy to reduce the spread of non-persistently transmitted plant viruses. Control strategies other than conventional treatments have thus to be developed either by the plantation of resistant material (see above), the exclusive propagation and commercialisation of PPV-free nursery plants accurately checked by reliable diagnostic methods, or the use of innovative strategies reducing PPV incidence in nurseries and orchards. One objective of this project will be to develop standards for PPV testing and certification of nursery stock among EU countries. A portion of the research effort is directed at understanding how PPV spreads in nursery blocks, and from orchard to orchard within growing regions. This information will be used to develop of warning systems, to monitor the arrival of the first PPV-viruliferous aphids, of epidemiological models and of disease management systems. New strategies for PPV control in nurseries and orchards will be tested alone or in combination. This will be the case of oil treatments in nurseries. Mineral oils have been shown to interfere efficiently with non-persistently transmitted viruses by altering the transmission process. Two teams involved in the SharCo consortium have performed preliminary experiments with encouraging results. Successfull new strategies will be part of the PPV risk management system proposed at the end of the project.
• Formulation of EU policies for sharka containment
The final practical achievement of SharCo will be therefore the formulation of guidelines for stone fruit tree cultivation and trade and the transfer of the knowledge and propositions to stakeholders: EU policy makers, plant protection services, extension personnel, nurserymen, fruit growers, and fruit industry representatives. Likewise, certification standards at the European level are needed to guarantee PPV-free production, transport, and sale of nursery stock and budwood; they will be discussed and improved with the help of the relevant agricultural extension services.