Occurrence in Tunisia of potato tuber necrotic ringspot disease (PTNRD) caused by variant PVY^NTN of Potato virus Y

S. Boukhris Bouhachem¹, N. Khamassy¹, L. Glais² and C. Kerlan^2*

¹ INRAT, Laboratoire de Protection des Végétaux, 2049 Ariana-Tunis, Tunisia
² INRA, UMR 1099 BiO3P, F-35650 Le Rheu, France

^*camille.kerlan@rennes.inra.fr

Accepted for publication 16/02/07

Potato tuber necrotic ringspot disease (PTNRD) (Beczner et al., 1984) is now found worldwide. The causal agent has been characterised as a variant of Potato virus Y (PVY), designated PVY^NTN. Most PVY^NTN isolates display a recombinant genome with exchanges of sequences in the HC-Pro, NIa and CP genes between PVY^N and PVY^O (Glais et al., 2002; Nie & Singh, 2003).

Figure 1: Distinct ringspots on the surface of a potato tuber cv Monalisa harvested in Sidi Bouzid. Sunken arches will then become necrotic with some cracks through the periderm.

In 2005, PTNRD symptoms were observed in autumn crops in Southern Tunisia. Many harvested tubers displayed superficial necrotic ring spots. Plantlets grown from a collection of these tubers tested positive for PVY when tested by ELISA using an anti-PVY antiserum (Inra-Fnpppt, Le Rheu, France). Eleven isolates were aphid transmitted from potato plantlets onto Nicotiana tabacum cv Xanthi. Isolates from tobacco were compared to well-characterized isolates representative of PVY strains and variants: PVY^N, PVY^NTN, PVY^NW and PVY^O (Glais et al., 2002). All isolates were maintained in a regulated (20°C ± 2°C), insect-proof greenhouse.

Figure 2: Agarose gel electrophoresis analysis of RT-PCR products amplified using a protocol based on the polymorphism in the P1 gene (Glais et al, unpublished). Lane E: SmartLadder (EuroGentec), lane 1: PVY^NTN-H, lane 2: PVY^NTNFrOrl, lanes 3-6: Tunisian isolates.

All Tunisian isolates tested in tobacco gave leaf distortion and vein necrosis, as did reference isolates of the three PVY^N subgroups. Tunisian isolates also reacted positively in ELISA with specific anti-PVY^N antibodies (Inra-Fnpppt, Le Rheu, France) and negatively with anti-PVY^O-C antibodies (Adgen, Ayr, UK). Hence biological and serological typing showed that none of the Tunisian isolates belonged to the PVY^NW variant since PVY^NW isolates are known to induce necrosis on tobacco and to react negatively with anti-PVY^N antibodies.

Four Tunisian isolates were characterized at the molecular level using RT-PCR protocols with specific primers, either based on the polymorphism in the P1 gene (Glais et al., 2001), or targeting the three recombination sites specific to PVYNTN isolates, RJ1 and RJ2 (Nie & Singh, 2003), RJ3 (Glais, unpublished). Tunisian isolates gave PCR products with sizes identical to those produced by the reference PVYNTN isolates, which demonstrated they are PVYNTN isolates with recombination events in HC-Pro/P3, CI/NIa and C-ter CP regions. This is the first report of PVYNTN isolates associated with PTNRD in Tunisia. PTNRD disease has never been previously documented in any potato growing area in Tunisia. Further studies are in progress to assess the incidence of the PVYNTN variant in both spring and autumn potato crops. Figure 3: Agarose gel electrophoresis analysis of RT-PCR products amplified using a protocol targeting the three recombination junctions, RJ1 and RJ2 (from Nie & Singh, 2003), RJ3 (from Glais, unpublished). Lanes E and 1-6 as Fig 2.

Acknowledgements

The authors would like to thank A. Najjar and M. Guillet for their excellent assistance and R. Kutcher for his help with English.

References

Beczner L, Horváth H, Romhányi I, Förster H, 1984. Studies on the etiology of tuber necrotic ringspot disease in potato. Potato Research 27, 339-352.

Glais L, Tribodet M, Kerlan C, 2001. Molecular detection of particular PVY isolates: PVY^NTN and PVY^NW. Proceedings of the 11^th EAPR Virology Section Meeting, Havlíčkův Brod – Třešť (Czech Rep.), 70-71.

Glais L, Tribodet M, Kerlan C, 2002. Genomic variability in Potato potyvirus Y (PVY): evidence that PVY^NW and PVY^NTN variants are single to multiple recombinants between PVY^O and PVY^N isolates. Archives of Virology 147, 363-378.

Nie X, Singh RP, 2003. Specific differentiation of recombinant PVY^N:O and PVY^NTN isolates by multiplex RT-PCR. Journal of Virological Methods 113, 69-77.

The British Society for Plant Pathology