Management of tomato viroids at World Vegetable Center headquarters

Seed-borne pospiviroids have become major quarantine concerns for the international movement of seeds of Solanaceous crops and their wild relatives. When tomato seed lots exported from World Vegetable Center (WorldVeg) Headquarters, Taiwan, were intercepted and some tested positive for the presence of pospiviroids, all distribution of tomato and pepper seed from the Center was suspended until the extent of the problem was known and strategies to manage them were developed. With technical assistance from the Taiwan Agricultural Research Institute (TARI), RT-PCR protocols for detection of pospiviroids in leaf and seed samples were adapted and optimized. In addition, the protocol for treating seeds by washing in 0.5 N HCl (15 min) followed by 10% (w/v) Tri-sodium phosphate (TSP) solution (1 h) was found effective at removing Pospiviroid contamination from the seed coat. A standard operating procedure (SOP) was adopted at the Center whereby 1) seed lots for multiplication or breeding, or for distribution abroad or in Taiwan are first cleaned of potential surface contamination and then samples (GROTERDAN1% of each seed lot) are tested by RT-PCR and those seed lots that test positive for pospiviroids are quarantined/destroyed, and 2) leaf samples are taken from approximately 10% of all the tomato seedlings grown and only those accessions or lines testing negative for pospiviroids by RT-PCR are allowed to be transplanted to the field. Columnea latent viroid (CLVd) and Pepper chat fruit viroid (PCFVd) were the predominant viroids detected. With adoption of the SOP the rates of viroid positive detection of tomato seed lots declined from 9.35 to 3.38% and 1.72% in 2016, 2017 and 2018, respectively. Co-infection of CLVd with PCFVd was found in only a few seed lots. From the results so far, the seed treatment and seed/seedling sanitation with monitoring by RT-PCR, appear to be effective in managing the viroids and reducing the positive detection rate in the tomato germplasm collection and breeding lines at WorldVeg.