VIRTIGATION partners Natural Resources Institute & Volcani Center have explored the role of seed transmission on viral disease spread to Europe.
Article by Tiziana Centofanti from the VIRTIGATION project
A crucial issue for import policies of seeds
Understanding the role of seed transmission is crucial in defining policies for importing horticultural seeds to Europe. In a recent report, scientists from VIRTIGATION partners Natural Resources Institute (NRI) and Volcani Center (VC) have delved into the complex issue of seed transmission, differentiating between seed-borne and seed-transmitted viruses. They focused their investigations specifically on the ToLCNDV and ToBRFV plant viruses. Thanks to an in-depth examination of these viruses in their VIRTIGATION research, including studies on cucurbits and tomatoes, the NRI and VC scientists could ascertain the role of seed transmission in viral disease spread of ToBRFV and ToLCNDV to Europe. These findings will enable EU plant health policymakers to refine horticultural seed import policies, ultimately contributing to more effective phytosanitary measures.
Seed transmission of ToLCNDV
Firstly, NRI researchers, together with the Indian VIRTIGATION international partner University of Agricultural Sciences Bangalore (UASB), investigated the seed transmission of ToLCNDV, a begomovirus affecting cucurbits in Europe with high genetic diversity enabling host adaptation. Previous research outside of VIRTIGATION had e.g. identified that an Indian strain exhibited 25% seed transmission in chayote, whereas a Spanish strain in melon was seed-borne but not seed-transmitted. The VIRTIGATION research conducted as part of the report also revealed inconsistent seed transmission results, likely influenced by virus strain and host species.
Moreover, NRI and UASB scientists analysed five Indian cucurbits for seed transmission. The scientists found the highest external infection rates in the bitter gourd crop (6%), with internal infection also peaking in bitter gourd (3.1%) and embryo infection limited to this species (2.5%). They did not observe any infections in the bottle gourd crop. Seedling germination and grow-out tests showed no evidence of seed transmission across cucurbits, aligning with Spanish findings that ToLCNDV is seed-borne but not seed-transmitted.
Seed transmission of ToBRFV
Secondly, VC researchers explored the seed transmission of ToBRFV, a devastating tobamovirus infecting tomatoes worldwide, even those resistant to other tobamoviruses. ToBRFV is primarily seed-borne, and contaminates the seed coat. While not found in the embryo, ToBRFV can infect seedlings during germination. Previous studies outside of VIRTIGATION show a wide range of seed-to-seedling transmission rates. In the studies of VC scientists, the lowest rate was 0.025%.
However, even low rates can lead to significant ToBRFV spread via mechanical transmission. Pest Risk Analysis considers seed transmission a high-likelihood entry pathway. EU plant health policymakers recommend RT-qPCR for seed testing with large sample sizes. Current EU regulations require differential sampling based on seed source country. Based on this report, VIRTIGATION VC researchers recommend that existing regulations for movement of seeds should be maintained.
Future prospects include developing new testing methods like large segment (LS-PCR) amplification, creating seed disinfection protocols, and potentially using LS-PCR to determine the infectivity potential of viral fragments on disinfected seeds. Due to the emergence of ToBRFV mutants that overcome resistant tomato varieties, regular seed testing of all new tomato varieties should continue, according to VIRTIGATION VC researchers.
Policy implications of VIRTIGATION findings
This VIRTIGATION report by partners NRI and VC underscores the nuanced nature of viral seed transmission. While ToBRFV demonstrates a low but significant rate of seed transmissibility, ToLCNDV’s seed transmission remains a subject of ongoing debate, with evidence suggesting significant variability based on virus strain and host plant. The project’s research underscores the importance of rigorous testing and monitoring, particularly for viruses like ToLCNDV where seed-borne presence does not always equate to transmission. Furthermore, the need for strain-specific assessments and the continuous evaluation of risk pathways, following recommendations by EFSA and EPPO, are crucial for adapting phytosanitary policies. Moving forward, a combination of vigilant seed sourcing, robust diagnostic testing, and comprehensive monitoring of imported plant materials will be essential to mitigate the risk of viral disease spread and protect European agriculture.
Read here the deliverable “Policy document on the role of seed transmission on viral disease spread to Europe” produced by VIRTIGATION partners NRI and VC, which describes their research made in more detail.
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