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Scientific Opinion on the risks to plant health posed by European versus non-European populations of the potato cyst nematodes Globodera pallida and Globodera rostochiensis
The Panel on Plant Health has delivered a scientific opinion on the different risks posed by European and non-European populations of the potato cyst nematodes (PCN) Globodera pallida and Globodera rostochiensis to solanaceous plants in the EU and on the effectiveness of current control measures. Although PCN, particularly G. rostochiensis, are widespread in the EU, crop damage is limited because breeders have been able to develop varieties that are resistant to the small number of genotypes that are present. These genotypes represent a minor subset of the gene pool and virulence that is present in South America. As new South American genotypes are very likely to have a similar potential for establishment and spread as existing European genotypes, the potato varieties currently grown in Europe will not be resistant to new virulent genotypes. As resistant varieties take a very long time to develop, the consequences of a new introduction of South American PCN would be major. The Panel therefore concluded that it is very important to maintain the current phytosanitary measures to prevent the entry of South American PCN. However, uncertainties over the effectiveness of the measures in Annex IVAI relating to place of production freedom and soil origin were noted, and the Panel identified additional risk reduction options for certain plants for planting (e.g. bulbs) and additional requirements to confirm the absence of PCN in places of production. The Panel also identified some problems with the existing control measures to reduce the spread of PCN within the EU. A thorough and well-coordinated EU-wide survey using standardized methods would be necessary to evaluate the need to maintain these measures. The monitoring of PCN populations should exploit new diagnostic techniques (e.g. mitochondrial DNA sequences) to ensure that the resistance available is deployed appropriately.
© European Food Safety Authority,2012
Following a request from the European Commission, the Panel on Plant Health was asked to deliver a scientific opinion on the health risks posed by European and non-European populations of the potato cyst nematodes (PCN) Globodera pallida and Globodera rostochiensis to solanaceous plants in the EU. In particular, the Panel was asked to determine the extent to which European and non-European populations of PCN differ in terms of their risk to the cultivation in the EU of plants of the family Solanaceae, primarily potatoes and tomatoes. It was also asked to provide an opinion on the effectiveness of the measures listed in Council Directive 2000/29/EC and Council Directive 2007/33/EC in reducing the risk to plant health posed by G. pallida and G. rostochiensis.
The Panel conducted the risk assessment following the general principles of the “Guidance on a harmonized framework for pest risk assessment and the identification and evaluation of pest risk management options” (EFSA Panel on Plant Health (PLH), 2010). The effectiveness of the current measures in place is evaluated in the section “Risk reduction options”.
After consideration of the evidence, the Panel reached the following conclusions:
With regard to the assessment of the risks to plant health posed by European versus non-European populations of the potato cyst nematodes Globodera pallida and Globodera rostochiensis:
G. rostochiensis is present in all Member States. G. pallida is present in all Member States except Denmark, Estonia, Latvia, Lithuania and Slovakia. According to Commission Regulation (EC) No 690/2008 of 4 July 2008, recognizing protected zones exposed to particular plant health risks in the Community, Finland, Latvia, Slovakia and Slovenia are protected areas for G. pallida. The PCN are well-characterized pests of potato, tomato and aubergine and are considered to be the most important nematode threat to potato production worldwide. The PCN originated in South America. Their initial introduction into Europe consisted of a small number of cysts that represent a restricted proportion of the gene pool and virulence present in South America. All PCN that are currently present in Europe, i.e. G. rostochiensis and G. pallida, represent a minor subset of the full biological diversity present in South America. The range of virulence of PCN present in South America is far greater than that present in European populations. Based on the recognition that non-European populations elsewhere except in South America have similar genetic diversity to those currently present in the EU, only the risk posed by PCN from South America has been assessed. Having evaluated the evidence for entry, establishment, spread and potential consequences, the Panel came to the following conclusions:
- Further entry from South America is assessed as very unlikely. This is because all the realistic pathways whereby PCN could enter the EU from South America have been closed for over 40 years by phytosanitary regulations and these regulations are very unlikely to be withdrawn as these pathways could also allow the entry of several harmful organisms listed in the annexes to the Plant Health Directive. Although the current phytosanitary regulations appear to have been effective in preventing the entry of PCN from South America through trade in plants for planting, vegetables, soil and agricultural machinery, and other pathways are improbable, the Panel considered that the effectiveness of the measures listed in Annex IVA1, sections 33 and 34, concerning place of production freedom and soil could be influenced by (a) what exactly is undertaken by the exporting country to justify the official statements written to confirm that the special requirements listed have been implemented and (b) the lack of consensus among EU Member States on whether bulbs are plants without roots. The Panel concluded that the concerns over the effectiveness of the Annex IVAI measures were sufficient to justify a score of medium uncertainty.
- No difference in the probability of establishment of European and non-EU populations (including those from South America) in Europe has been identified. A new viable PCN population introduced within Europe, either with infested soil or with infested tubers, would result in a primary introduction of at least a single cyst within a farmer’s field. Detection of this introduction (e.g. a new virulent strain) in the receiving country is likely to occur only after several years of potato cropping, when the infestation has grown in both area and density and reaches detection level. By then the virulent strain would have been distributed widely by infested seed potato lots from the original primary infested field. There is no information to suggest that South American populations of PCN would be less able to survive within the EU than populations that are currently present. All of the “secondary” infested fields would also have infestations below the detection threshold of any statutory detection method. New infestations take many years to develop to a detectable level. Thus, it is likely that, by the time a new introduction has finally been detected, it would be too late to allow eradication, even if this were possible. All currently available control measures are ineffective and very unlikely to kill more than a fraction of the population present in the field. There is little uncertainty about the described scenario. In the last 100 years numerous countries have tried unsuccessfully to prevent the introduction of PCN. Even countries with very tight restrictions, and those that have attempted eradication, have confirmed infestations of PCN, e.g. the USA (Idaho infestation) and Australia.
- It is not possible to remove all threat of PCN contamination associated with different methods of spread, whether by natural means (e.g. wind, water, animals) or with human assistance (e.g. soil waste, seed potato, machinery, etc.). Spread can be minimized, especially when all possible vectors of dispersion are addressed, but will still occur. The probability of spread of PCN in Europe was therefore assessed as very likely with low uncertainty because there is ample evidence from several EU Member States of failed attempts to prevent the spread of PCN to new potato-growing areas. All possible vectors of spread have been addressed in research, but tested solutions have proven not to be absolutely failsafe. There is also no reason to believe that the spread of EU and non-EU populations would differ.
- The Panel recognized that only a limited range of control options is currently available for PCN. The use of resistant cultivars resulted in a decline in PCN populations and is recognized as the most important and effective control option in Europe. It was assessed that further introductions of PCN from South America are almost certain to have different virulence characteristics from the PCN currently present in Europe. This will lead, in time, to the currently available resistance against PCN (which has been bred against populations currently present in Europe) being overcome. The impact on the potato is therefore expected to be major. The associated uncertainty is low, because it has been demonstrated unequivocally that South American populations of both species of PCN are virulent to the resistance sources used in European cultivars and that selection for virulence can occur. The PCN populations originating from South America are considered a potential threat to potato cultivation control in Europe if introduced here.
With regard to risk reduction options, the Panel evaluated the phytosanitary measures formulated in Council Directives 2000/29/EC and 2007/33EC and identified additional risk reduction options where relevant.
Risk reduction options reducing the probability of introduction
Risk reduction options reducing the probability of introduction of non-European populations of PCN are essential to the protection of potato production in the EU territory, because the control programmes for PCN largely rely on the resistance of potato varieties to the populations of PCN currently known in the EU.
Concerning the risk reduction options to reduce the probability of entry of PCN, the Panel concludes that the set of import prohibitions and special requirements for the import of specific commodities, currently set out in Council Directive 2000/29/EC, largely cover all the identified pathways for introduction of PCN into the EU. Additional risk reduction options would be necessary to reduce the probability of introduction of PCN carried on bulbs, tubers (other than Solanaceae), rhizomes and corms intended for planting. The Panel considers that the special requirements for the import of specific commodities are technically feasible, but their effectiveness depends on the methods that have been employed and on the quality control of the application of these methods. The Panel identifies the need for the formulation of additional requirements to specify the criteria in official statements concerning the absence of PCN in a place of production.
Risk reduction options reducing the probability of spread
The Panel concludes that there are no risk reduction options available that would completely prevent the spread of PCN. However, the combined implementation of available risk reduction options, based on obligatory intensive soil sampling and testing of fields prior to growing seed potatoes and other plants for planting, minimizes the rate of spread. As there is still a considerable proportion of fields without PCN or with PCN infestation levels below the detection threshold of the sampling method in the Member States, the Panel concludes that continuation of officially implemented risk reduction options is justified. The Panel notes various points of concern with the risk reduction options currently implemented to reduce the probability of spread of PCN within the EU:
- The reduction in soil sampling rate under specified conditions provided by derogation in Council Directive 2007/33/EC may enhance rather than reduce spread of PCN.
- The special requirements to prevent spread of PCN to the protected zones, formulated by Annex IVB (23) of Council Directive 2000/29/EC, have become ineffective since Council Directive 69/465/EEC was repealed by Council Directive 2007/33/EC.
- The limitation of the range of plants with roots addressed by Council Directive 2007/33/EC, compared with Council Directive 2000/29/EC, may have a small effect on the probability of spread of PCN in the short term, but it may increase the probability of spread of PCN in the longer term (15–20 years).
- The formulation of categories of host plants in Annex I-2(b) of Council Directive 2007/33/EC is highly unclear and, on the basis of this text, spread of PCN would be enhanced rather than reduced.
- The absence of requirements for movement of soil, in particular soil known to be infested with PCN, in both Council Directive 2000/29/EC and Council Directive 2007/33/EC enhances the probability of spread of PCN.
Risk reduction options reducing the infestation of PCN in growing crops
The use of potato cultivars resistant to the PCN populations present in the field is the keystone of the system of control measures to reduce PCN infestation in growing crops. However, this is true only if the introduction of new PCN populations continues to be prevented, because these cultivars have been developed in relation to the populations of PCN currently present in the EU. The Panel concludes that the use of the standard scoring notation table (see sections 4.4.1 and 4.4.2) as required by Council Directive 2007/33/EC, rather than the measured level of resistance, prevents the optimal use of resistant varieties to reduce PCN infestation in growing crops.
Based on the findings of this opinion, the Panel has drawn the following overall conclusions:
- It has been shown that nematode populations can respond to selection through repeated use of resistant cultivars. It is therefore important to continue to monitor and characterize populations of nematodes present within the EU so that available resistance is deployed appropriately.
- Tools for characterization of populations of PCN are now starting to become available, mainly based on differences in mitochondrial DNA (mtDNA) sequences. Further research on the development and validation of such tools, and on the molecular basis of virulence in PCN, would help management of PCN within the EU.
- Additional risk reduction options would be necessary to reduce the probability of introduction of PCN carried on bulbs, tubers (other than Solanaceae), rhizomes and corms intended for planting, and specifically to include bulb crops in the group of commodities for which an official statement that the place of production is known to be free from PCN is required.
- As the detection of PCN in fields is strongly affected by the detection methodology, a specification of the required methods for soil sampling and testing is necessary to support the official statements on the absence of PCN in a place of production, as required for plants with roots intended for planting.
- If the standard scoring notation table is maintained, it is recommended that both the measured partial resistance level and the standard score be provided by the Member States.
- A thorough and well-coordinated EU-wide survey is needed to determine the distribution of G. rostochiensis and G. pallida in terms of the proportion of infested fields and area within each Member State as a basis for evaluating the need for maintaining risk reduction options to prevent the spread of PCN.
Potato cyst nematodes, Globodera spp., European populations, non-European populations, Solanaceae, genetic variation, virulence