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Tomato chlorosis virus infecting Luffa cylindrica and Citrullus lanatus in China.

Sponge gourd (Luffa cylindrica) and watermelon (Citrullus lanatus) are important cash crops in China. In September 2015, interveinal yellow spots and chlorosis, suspected to be caused by the tomato chlorosis virus (ToCV; genus Crinivirus), were observed on sponge gourd and watermelon plants in six greenhouses in the cities of Shouguang, Dezhou, and Taian (2 greenhouses in each city) of Shandong Province. The incidences of the disease in sponge gourd and watermelon greenhouses were 10% to 20%. To identify causative pathogens, 20 sponge gourd and 15 watermelon samples were collected from cucurbit plant facilities in Shandong Province, China. Total RNA was extracted from the samples using RNA simple Total RNA kit (Tiangen Biotech Co., Ltd., Beijing, China) according to the manufacturer's protocol. Reverse transcription-polymerase chain reaction (RT-PCR) of ToCV was performed using To-CP-forward (ATGGAGAACAGTGCTGTTGC)/To-CP-reverse (TTAGCAACCAGTTATCGATGC) primer pair (Hirota et al. 2010). DNA fragments of approximately 780 bp were detected in all sponge gourd and watermelon samples. The fragments were inserted into pMD18-T vector (Takara, Shiga, Japan), which was subsequently transformed into Escherichia coli DH5α. Sponge gourd (n=1; ToCV-sponge gourd) and watermelon (n=1; ToCV-watermelon)-positive samples were selected for Sanger sequencing. BLASTN comparison of the sequencing results confirmed the presence of ToCV. The sequencing results were processed using DNAMAN version 6.0 (Lynnon Biosoft, USA) and submitted to the GenBank database (https://www.ncbi.nlm.nih.gov/). The phylogenetic tree based on ToCV coat protein (CP) was constructed using amplified ToCV-sponge gourd, ToCV-watermelon, and ToCV representative sequences in GenBank database. According to the results, the ToCV sponge gourd and watermelon sequences belonged to an independent branch with the Chinese ToCV isolate (KC812619). Sequence analysis based on nucleotide sequences of ToCV CP demonstrated that ToCV-sponge gourd and ToCV-watermelon isolates shared the highest nucleotide sequence identity of 99.7% with the Chinese isolate (KC812619). To assess the transmissibility of ToCV, virus-free whiteflies (Bemisia tabaci) (n = 30) were placed for one day on ToCV-infected sponge gourd and watermelon plants for virus acquisition. Thereafter, whiteflies were transferred onto the virus-free sponge gourd (cv. 'Changlv', 4-leaf-stage, n = 6 for each of the control, ToCV treatment) and watermelon (cv. 'ZaoJia 8424', 4-leaf-stage, n = 6 for each of the control, ToCV treatment) seedlings for one day. Three weeks later, all plants from tested group showed same symptoms as those observed in the greenhouses, whereas plants in the control group were symptom-free. RT-PCR analysis confirmed the ToCV infection in sponge gourd and watermelon plants, whereas control plants were found uninfected. ToCV infection in sponge gourds and watermelons has not been reported previously. To the best of our knowledge, this is the first report of sponge gourd and watermelon being natural hosts of ToCV worldwide. We believe that spread of ToCV in cucurbits needs attention.