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Potato Tuber Necrotic Ringspot Disease Occurring in Japan: Its Association with Potato virus Y Necrotic Strain

Unfamiliar necrotic symptoms on or within potato tubers of cultivars Nishiyutaka and Dejima were observed in Nagasaki prefecture, Japan, in 1992. Symptoms were typically on the surface of the tuber, which either protruded at first and then became sunken, or showed necrotic spots, with necrosis within the tubers. Symptoms sometimes appeared at harvesting but more often appeared after storage for several months. Investigations revealed that the causal agents of the disease were isolates of Potato virus Y necrotic strain (PVY^NTN) and the disease was potato tuber necrotic ringspot disease (PTNRD), previously reported in Europe and Lebanon. Five potato cultivars were inoculated with an isolate from necrotic tubers. The highest percentage of progeny tubers showing PTNRD was found in cv. Nishiyutaka (23.3%). In contrast, cvs. Shima-bara, Mayqueen, and Danshaku showed a low percentage of PTNRD. Additional potato tubers with PTNRD were also observed after storage of the tubers. To investigate the relatedness between isolates of PVY^NTN and of necrotic strain PVY^N, previously isolated in Japan, Nishiyu-taka was inoculated with an isolate of PVY^N, which also induced PTNRD. Nucleotide sequences of coat protein (CP) genes of six PVY^NTN isolates were determined. The CPs were 267 amino acids in length, with a substitution of one or no amino acid among each of the six isolates. The phylogenetic relationship based on nucleotide sequences of CP genes showed that these six PVY^NTN isolates clustered together with PVY^N isolates. This is the first report of PTNRD caused by PVY^NTN isolates in Asia.