Recent emergence of seed-borne viruses and viroids on tomato, seed health tests and their implications in global seed trade

Tomato is one of the most important and widely grown vegetables in the World. With an increasing trend in offshore hybrid seed production and global seed trade activity, the risk of introducing a seed-borne pathogen from a different country with seed as a pathway is high if no rigorous seed health testing program is implemented. For some common seed-borne viruses, such as Tobacco mosaic virus (TMV), Tomato mosaic virus (ToMV) and Pepino mosaic virus (PepMV), standard seed health assay methods are available. For those emerging seed-borne viruses and viroids, including Tomato mottle mosaic virus (ToMMV), Tomato brown rugose fruit virus (ToBRFV), Potato spindle tuber viroid (PSTVd) and several other tomato-infecting pospiviroids, there is an urgent need to understand the mechanism of seed transmission and to develop appropriate seed-health assays. To achieve a meaningful risk assessment, a sensitive laboratory test needs to be complemented by an appropriate bioassay. This is especially important for those seed lots that are treated to remove surface contamination. Recent outbreaks of ToBRFV and pospiviroids on greenhouse tomato in the US and Mexico, highlight the importance of including those emerging viruses and viroids in seed health testing. For seed-borne viruses, the most common technique for seed health test is a serological method (e.g., ELISA) followed by a bioassay to confirm the infectivity of virus particles in a seed lot. For tomato-infecting pospiviroids, with multiple species and greater genetic diversity, there is still a debate as to whether all known tomato-infecting pospiviroids or just some of them should be included. For those seed health assay methods that are sequence-based techniques, genetic variability should be considered in designing the primers that will offer a sensitive detection, with no cross-reaction to other related species. These challenges are addressed, and suggestions for developing meaningful seed health programs are made.