A Regulated Pathogen Challenges Global Tomato Seed Shipments

- Ravi Chitrampalam

Potato Spindle Tuber Viroid (PSTVd) is one of the top challenges to international tomato seed shipments due to strict phytosanitary regulations by importing countries. It reduces both the yield and marketable quality of the fruit. Seed is believed to be the source of PSTVd outbreaks in tomato and therefore, many countries impose rigid phytosanitary regulation on this pathogen. In addition to PSTVd, several other pospiviroids (CEVd, CLVd, MPVd, PCFVd, TASVD, TCDVd and TPMVd) cause disease in tomato. These are all regulated in part or in total by some countries.

Methods to precisely and quickly detect pospiviroids directly from seed would be of great help to the seed industry. However, there are challenges in detecting these pathogens from seeds. Unlike viruses, viroids do not encode any proteins. Therefore, classical ELISA methods that are regularly used to detect viruses are not useful for finding viroids. Molecular methods are the best choice for quick detection. Several molecular methods are published in the literature to detect pospiviroids from tomato. Most of the publicly available methods are useful to detect viroids from infected tissues but not from seeds. Only a few methods are currently available in the public domain to detect pospiviroids from seeds, but they are either not validated for large scale testing or are not user friendly. A phytosanitary requirement from one country requires the testing of 20,000 seeds subdivided into 50 – 400 seed subsamples. This is a very expensive and time-consuming procedure.

Scientists from the global seed industry, including Eurofins BioDiagnostics Inc., are collectively working to develop a detection method that not only meets phytosanitary requirements but also serves as an industry standard for internal quality assurance testing. The new method will be subjected to rigorous validation before it’s accepted for routine testing. Until the new protocol is available, several companies currently use their own internally developed protocols for pospiviroid testing. Eurofins BioDiagnostics Inc. has optimized a protocol for efficiently detecting pospiviroids from both tomato and pepper seeds. These molecular methods do not reveal any information on the viability and virulence of the pathogens.

Seed lots with non-viable or non-pathogenic strains could be detected as positive and thus give a false positive result. To overcome this issue, scientists from both academia and industry are doing extensive research to develop a bioassay and/or pathogenicity assay to determine the viability and virulence of these un-culturable pathogens.