First Report of Potato Tuber Necrotic Ringspot Disease Caused by PVY^NTN in Portugal

During the last 2 years, potato (Solanum tuberosum L.) tubers displaying superficial necrotic arcs and rings were found in central Portugal. These symptoms increased during storage, and diminished tuber quality of ware (fresh-market) potatoes; however, no internal necrosis, which is typical for infections caused by tobacco rattle virus or potato mop top virus, was observed. The symptoms led to the preliminary diagnosis of potato tuber ringspot disease (PTNRD), caused by a tuber necrosis (TN)-inducing isolate of the tobacco veinal necrosis strain group of potato virus Y (PVY^N) that was named PVY^NTN. The occurrence of PVY^NTN has been reported by a number of European countries. Suspect PTNRD tubers of the cv. Monalisa were obtained from several Portuguese potato growers and were tested with polyclonal antibodies (pabs) that reacted generally with PVY, and with monoclonal antibodies (mabs) raised against PVY^N. Serogical tests were carried out in a double antibody sandwich (DAS) enzyme-linked immunosorbent assay (ELISA) with pabs and in a triple antibody sandwich (TAS) ELISA when mabs were used. As a result, the tubers were found to be infected with a virus isolate belonging to the PVY^N strain group. Since PVY^NTN cannot be distinguished serologically from other members of the PVY^N strain group due to the similarities of their coat proteins (1), reverse transcription-polymerase chain reaction combined with immunocapture was applied for diagnostic purposes. The olignucleotide primers used were located in the 5' non-coding region at nucleotide 103 and in the adjacent P1 protein gene coding region at position 919. This primer pair can be used to distinguish PVY^NTN from other members of the PVY^N strain group (2). Tests were carried out with plant sap from tubers and from plants grown from eye-cuttings and also from tobacco plants that were inoculated with plant sap of these potato tubers and plants. Control samples included sap from un-infected tobacco plants and from tobacco plants infected with a PVY^N isolate and with the PVY^NTN type strain "Hungary". The expected amplification product of 835 bp appeared in the agarose gel with samples originally obtained from the tubers and with the PVY^NTN control but not with the PVY^NTN control, indicating that the tuber symptoms in potato cv. Monalisa were caused by infections with PVY^NTN. This is the first report of the occurrence of PVY^NTN in Portugal. References: (1) T. Dalmay and E. Balazs. Nucleic Acids Res. 18: 6721, 1990. (2) H. L. Weidemann and E. Maiss. Z. Pflanzenkr. Pflanzenschutz 103:337, 1996.