First Report of Olive Mild Mosaic Virus in Imported Tulips (Tulipa gesneriana) in Korea.

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Tulip cultivation in Korea primarily uses imported bulbs due to the absence of domestic production. To ensure safety and sustainability, Korean authorities have implemented strict phytosanitary measures for five viruses: arabis mosaic virus, tobacco necrosis virus, tobacco ringspot virus, tomato black ring virus, and tomato bushy stunt virus. In April 2021, 86 tulip plants presented symptoms such as chlorotic mottle, mosaic, streak, stripe, yellowing of the leaves, and color breaking on flowers. These samples were collected to investigate the incidence of viruses in four Korean provinces (Gangwon, Gyeongbuk, Gyeongnam, and Chungnam). The leaves and petals from each individual sample (10 mg each) were pooled and ground using liquid nitrogen. Total RNA was extracted using a Maxwell® 16 LEV Plant RNA Kit (Promega, Madison, USA). A cDNA library was constructed using TruSeq Standard Total RNA with Ribo - Zero (Illumina, San Diego, USA) and sequenced on an Illumina NovaSeq 6000 platform (Macrogen, Seoul, Korea) with 100-bp paired-end reads. Trinity software identified tulip breaking virus (TBV), tulip virus X (TVX), and lily symptomless virus (LSV), which are known to occur in Korea (Bak et al. 2023) by de novo assembly of 628 million reads into 498,795 contigs. The contigs were annotated as previously described (Bak et al. 2022). Moreover, a contig (ON758350) related to olive mild mosaic virus (OMMV; genus Alphanecrovirus, family Tombusviridae) was identified through BLASTn analysis. This contig had a 99.27% nucleotide (nt) identity to OMMV PPO-L190209 (KU641010), which was assembled from 201,346 reads and spanned 3,713 bp. To confirm the presence of OMMV, a primer pair (5'-GAATGTCTGGCGTTAAGCG-3'/5'-GTGTCCTGCGCATCATACAC-3') was designed to amplify a 797-bp fragment of the coat protein gene. In RT-PCR, 31.4% (27/86) of samples were positive for OMMV and coinfected with TBV or TBV+LSV. Coinfection with TBV led to chlorotic mottling and stripes, whereas triple coinfection with TBV+LSV produced distinct yellow streaks and mosaic within the lesion boundaries. In contrast, solely TBV infection did not produce such symptoms. The samples infected with OMMV were exclusively collected from Gangwon and Gyeongnam. In each province, an RT-PCR amplicon was cloned, and subsequently sequenced (Bioneer, Daejeon, Korea). The obtained sequences were named CC (OM243091) and GS (OM243092), and they shared 98.6% and 98.9% identity with PPO-L190209 (KU641010), respectively. A bioassay was conducted using a leaf infected with OMMV CC and TBV to inoculate 13 indicator species in triplicate, including Capsicum annuum, Chenopodium amaranticolor, C. quinoa, Cucumis sativus, Nicotiana benthamiana, N. clevelandii, N. glutinosa, N. occidentalis, N. rustica, N. tabacum, Solanum lycopersicum, Tetragonia tetragonioides, and Tulipa gesneriana. The RT-PCR revealed positivity only for OMMV in the upper leaves of N. clevelandii, while all other species were negative with no symptoms. To our knowledge, this is the first report of OMMV occurring in tulips grown from imported bulbs in Korea, with no other known natural hosts such as olive tree (Cardoso et al. 2004), spinach (Gratsia et al. 2012), and corn salad (Verdin et al. 2018). The Korean OMMV isolates exhibited a high nt identity with the foreign isolate, and the samples were collected from farms that rely entirely on bulb imports for cultivation. These suggest that the outbreak of OMMV was likely caused by imported bulbs.