First Report of Sweet potato virus G Infecting Sweet Potato in Taiwan.

Abstract

Sweet potato, Ipomoea batatas (L.) Lam., is an important root crop grown mainly in the counties of Changhua, Yunlin, Tainan, and Pingtung in Taiwan where Sweet potato feathery mottle virus (SPFMV) and Sweet potato latent virus (SPLV) have been reported. Commercial sweet potato grown in Nantou in 2009 and in Hualian in 2010 exhibited downward leaf curling and vein clearing, indicative of viral infection, yet symptoms were distinct from those caused by SPFMV, SPLV, or mixed infection of both viruses. Total RNA was extracted from two symptomatic plants from each county with RNeasy Plant Mini Kit (Qiagen, Hilden, Germany) and analyzed by reverse transcription (RT)-PCR using the potyvirusdegenerate primer Hrp5 (1) and oligo-dT18 with BamHI site at the 5' end (5'-GGATCCTTTTTTTTTTTTTTTTTT-3'). Two healthy plants served as negative controls. An approximately 1.5-kb amplicon covering the region from the 3'-end of the nuclear inclusion protein b (NIb) gene to the 3'-untranslated region (3'-UTR) was amplified from all symptomatic plants, while the healthy controls remained negative. Subsequently, one sample from each location was cloned and sequenced (GenBank Accession No. HQ171932-TW1 [Nantou] and JN205346-TW2 [Hualian]). Based on sequence comparison, the two isolates shared only 86.7% nucleotide identity. BLAST analysis of the CP gene of the isolate TW1 revealed 99% nucleotide identity with the corresponding sequence of Sweet potato virus G (SPVG)-CH2 from China (Z83314). Isolate TW2, however, only shared 86% nucleotide identity with SPVG-CH2, indicating isolate TW2 is genetically different from other isolates and probably represents a new strain of SPVG. The presence of SPVG was further confirmed in symptomatic plants by indirect ELISA using SPVG antiserum developed by Y.-H. Cheng of the Agricultural Research Institute. Since co-infection of different viruses in sweet potato can cause severe leaf symptoms and significant yield reduction (3), a preliminary field survey was also conducted to determine the extent of co-infection with more than one potyvirus using three different primer pairs, SPVGup (5'-ACCGAGCTTTACCCCAGGTAGAGAG-3')/SPVdw (5'-CGCGCAAGACTCATRTCAGTCAAAT-3') for SPVG, FM16 (5'-GAATTTAAAGATGCAGGTGTGAAC-3')/FM895 (5'-GAGGTTATGTATATTTCTAGTAAC-3') for SPFMV, and L166 (5'-GACAGAGATATCAACACTGGCACC-3')/L841 (5'-TCCAAGTAGTGTGTGTATGTTCCG-3') for SPLV. Forty-six of 128 (36%) sweet potato samples collected from Nantou, Hualian, Yunlin, Tainan, and Chiayi counties during 2010 and 2011 tested positive for SPVG. Of the 46 samples that tested positive for SPVG, six were co-infected with SPLV, 19 were co-infected with SPFMV, and two were co-infected with all three viruses. Of the samples that tested negative for SPVG, 10 were infected with SPLV, eight were infected with SPFMV, and two were infected with both SPLV and SPFMV. To date, SPVG has been detected in China, the United States, Peru, Egypt, Ethiopia, Zimbabwe, South Africa, Spain, Java, New Zealand, Hawaii, French Polynesia, and Easter Island (2). To our knowledge, this is the first report of SPVG infecting sweet potato in Taiwan. SPVG could become a new and potentially serious threat to sweet potato production in Taiwan.