MANAGEMENT OF NEMATODES AND VIRUS DISEASES AFFECTING POTATO AND GRAIN CROPS
Location: Biological Integrated Pest Management Unit
Title: Serological properties of ordinary and necrotic isolates of potato virus Y: a case study of PVYN misidentification
Submitted to: American Journal of Potato Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 27, 2009
Publication Date: September 3, 2009
Citation: Karasev, A.V., Nikolaeva, O.V., Hu, X., Sielaff, Z., Whitworth, J.L., Lorenzen, J., Gray, S.M. 2009. Serological properties of ordinary and necrotic isolates of potato virus Y: a case study of PVYN misidentification. American Journal of Potato Research. 87:1-9. Available: http://www.springerlink.com/content/04u8141225868w72/
Interpretive Summary: Potato virus Y is the most economically important disease affecting the US seed potato industry. In recent years several new strains of the virus have been discovered in the US many of which cause severe necrosis in potato and tobacco and these are a threat to commercial production. State and federal regulatory agencies, and their equivalents in international trade partners, test for these necrotic strains of PVY strains using antibody based lab tests that are relatively quick and simple. We discovered a new variant of PVY that is identified as a necrotic strain by these antibody based lab tests, but this variant actually poses no threat since it does not cause necrosis in the host plant. However, detection of this virus variant has led to several shipments of potatoes being refused at border checkpoints and/or being destroyed rather than being sold. In this paper we characterized the new virus variant and determined that a minor change in one virus protein allows the virus to react with the antibody currently used to detect necrotic strains of PVY. This simple change in one protein does not affect any other property of the virus. We have tested other commercially available antibodies and identified one that does not identify this variant as a necrotic virus, but it does recognize the real necrotic viruses as such. A simple change in testing procedures used by regulatory agencies can avert these erroneous results and facilitate trade of potatoes across state and international boundaries.
In the course of a multi-year survey of PVY incidence and diversity in the U.S. seed potato crop, an unusual PVY variant was identified in low but significant levels in multiple states. This variant, PVYO-O5, was initially detected by a commercially available PVYN-specific monoclonal antibody, 1F5. This antibody is widely used by U.S. Seed Certification programs to test for PVYN and is one of two antibodies designated by the North American Plant Protection Organization (NAPPO) for pre-shipment testing of tuber lots that are to be transported between countries. Consequently, PVYN positives identified by the 1F5 antibody have triggered quarantine actions, prevented cross-border shipments and impacted trade. Here, we demonstrate by a variety of methods that the PVYO-O5 is a variant within the ordinary PVY strain (PVYO). Specifically, the PVYO-O5 variant likely arose due to a single amino acid substitution within the capsid protein. This variant does not induce vein necrosis in tobacco or tuber necrosis in susceptible varieties of potato, furthermore, it is identified by RT-PCR based diagnostics as PVYO and it has a typical PVYO genome sequence. We demonstrate that another PVYN specific monoclonal antibody, SASA-N, recognizes an epitope distinct from that recognized by 1F5, and correctly identifies the PVYO-O5 variants as belonging to the PVYO serotype. Since the PVYO-O5 variant is present in many seed producing states and misidentification of PVYO-O5 as PVYN/NTN has clear quarantine implications for export shipments of potato, the limitations of the commercially available monoclonal antibodies should be considered in any certification or phytosanitary testing program.