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 PSTVd status for USPG

The United States Potato Genebank (USPG) has had a rigorous Potato spindle tuber viroid (PSTVd) testing scheme for decades, using Agdia dot-blot Nucleic Acid Hybridization on small seedlings at both ends of seed increase grow-outs of all  accessions.  In 2020, some colleagues in other programs reported false negative tests on small in-vitro plantlets.  At the same time, we noticed that seed increases in 1992 and 1994 had several positives and became suspicious of the negative status of their seed increase cohorts.  When those cohorts were grown at an off-site quarantined greenhouse to much larger seedlings (about 20 cm tall), a few reputed negatives tested positive.  Accessions in the entire USPG collection of about 5,000 seedlots have been grown for decades in complicated overlapping cohorts in greenhouse and screenhouse seed increase grow-outs.  Research stocks were also present.  When false negatives are corrected to positives, records can be used for some contact tracing of potential spread can be done, but that would not be perfectly reliable. 

So the reality at USPG is:  We have no reason to posit widespread PSTV infection.  Since it appears that PSTVd detection sometimes failed in many of the years of testing, and perhaps even recently, we cannot be sure of the PSTVd status of most seedlots.

This suggests halting distributions of seedlots is needed until we have developed and deployed improved standards of establishing PSTVd negative testing status in such seedlots, validated testing, and having retested the whole seed collection or sets of seedlots as they are ordered.

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John B Bamberg
USDA/ARS Geneticist, Plants
US Potato Genebank (USPG)
(920) 743-5406

The mission of the USPG is to facilitate improvements in the potato of the future by promoting the use of valuable exotic genes found in wild potato germplasm. Small tubers are typical of wild potato species, but they represent a veritable treasure chest of genetic diversity for potentially useful traits that may be bred into new varieties. These new varieties must be able to overcome the challenges of pests and stresses with less dependence on chemical fertilizers, insecticides and fungicides. The USPG is doing this through a 5-fold approach: Acquisition, Classification, Preservation, Evaluation and Distribution of potato germplasm.