S.02. Transboundary movement of plant viruses and their potential spread threat by native vectors: implications under WTO regime

Viral diseases assume special importance when they are seed-borne, seed-transmitted and also transmitted by vectors leading to field spread. Even a low rate of virus transmission through seed can lead to severe yield losses if the insect vectors in the field are active. More than 100 viruses are now known to be carried through seeds of different plant species belonging to different families and spread through insect vectors, mainly aphids, and in certain cases through nematodes, beetles and fungi. Therefore, exchange of seed material at international level could play a key role in the long-distance dissemination of a destructive virus or its virulent strain. One such example is the introduction of Peanut stripe virus into the US and India. With the increasing global seed trade and exchange, such possible introduction of seed-transmitted viruses needs to be carefully controlled. With the seed trade now falling under the gamut of international agreements of the World Trade Organization (WTO) as well as with the removal of quantitative restrictions in imports since 2001 in India, the possible introduction of seed-transmitted viruses has been enhanced. This calls for stringent quarantine processing. As per the Plant Quarantine (Regulation of Import into India) Order, 2003 (PQ Order) issued under the Destructive Insects and Pests Act, the Directorate of Plant Protection Quarantine and Storage (DPPQS) under Ministry of Agriculture is responsible for enforcing quarantine regulations and also for making rules for quarantine inspection and disinfestation of any article or class of articles. Import of bulk material for sowing/planting purposes are authorized only through five of the twenty-nine Plant Quarantine Stations of DPPQS. The National Bureau of Plant Genetic Resources, the nodal institution for exchange of plant genetic resources has been empowered under PQ Order (2003) to handle quarantine processing of germplasm and transgenic planting material imported for research purposes in the country by both public and private sectors.

The size of the consignment/sample received is very critical in quarantine from processing point of view. In case of bulk consignments, a proper sampling of the lot is to be done as per the norms for ensuring an effective processing of the material through batch testing. In case of small but precious samples of germplasm extreme precaution is needed to ensure that the result obtained in the tested part is not a false positive or a false negative. A number of techniques ranging from biological to molecular are being used for reliable detection of plant viruses within a short span of time in a given amount of sample during quarantine processing. This requires an antisera bank for exotic viruses and a database on sequences of viruses specific primers, and so also a repository of seeds of indicator hosts for easy access to quarantine officials. Besides, if sample size is very small as in case of germplasm, it is not practical to directly test the seeds as a part of the samples has also to be released to the indenter. It thus takes a crop season to release the harvest only from the indexed virus-free plants from post-entry quarantine nurseries.

Adopting a workable strategy, a number of exotic viruses have been intercepted in imported germplasm which includes the ones yet not reported from India viz., Barley stripe mosaic virus on Hordeum vulgare and Triticum spp.; Broad bean stain virus on Vicia faba, Cherry leaf roll virus (CLRV) on Glycine max and Phaseolus vulgaris; Cowpea mottle virus (CPMoV) on Vigna unguiculata, Peanut stripe–virus [PStV = Bean common mosaic virus (BCMV)] and–Peanut mottle virus (PeMoV) on Arachis hypogaea, Raspberry ringspot virus (RpRSV) and Tomato ringspot virus (ToRSV) on G. max. Besides, nine viruses viz., Alfalfa mosaic virus (AMV) on P. vulgaris and V. radiata; BCMV on G. max; Cowpea aphid borne mosaic virus (CABMV) on G. max and V. radiata; Cowpea mosaic virus (CPMV) on V. radiata; Pea seed-borne mosaic virus (PSbMV) on V. faba; Southern bean mosaic virus (SBMV) on G. max and P. vulgaris; Soybean mosaic virus (SMV) on P. vulgaris; PStV and Tobacco streak virus (TSV) on G. max, Tomato black ring virus (TBRV) on P. vulgaris and V. unguiculata are not known to occur on the hosts in India on which they were intercepted. Four viruses viz., Bean common mosaic necrosis virus (BCMNV), Bean yellow mosaic virus (BYMV), Cucumber mosaic virus (CMV) and Tobacco ringspot virus (TRSV) are known to be present in India. However, there are reports of worldwide distribution of different virulent strains of these viruses. Even though many of the intercepted viruses are not known to occur in India, their potential vectors do exist such as nematodes for CLRV, RpRSV, TBRV, TRSV and ToRSV, beetles for CPMV, CPMoV, SBMV; aphids forAMV, BCMV, BCMNV, BYMV, CABMV, CMV, PeMoV, PSbMV, PStV, SMV; thrips for TSV and the congenial conditions also do exist for these vectors to multiply, disseminate and spread the destructive exotic viruses/strains. On the contrary a vector or its biotype, which are able to transmit the native viruses more efficiently, may also be introduced along with the imported planting material.. The recent inadvertent introduction of Bemisia tabaci biotype B is such an example that has spread the Tomato leaf curl virus in a rampant form in Karnataka and Andhra Pradesh.

This highlights the importance of quarantine processing in excluding the transboundary movement of viruses and their vectors. Under the WTO regime the Sanitary and Phytosanitary Agreement stipulates internationally accepted standards for quarantine and phytosanitation to be adopted in trade of agricultural commodities. Since viruses are not easy to be controlled by chemical or physical means, adopting the reliable techniques with an appropriate strategy for their detection would go a long way in ensuring virus-free trade and exchange of germplasm.