The nucleocapsid protein gene of a tospovirus infecting watermelon in India was cloned and sequenced. Sequence analyses showed that the gene was most closely related to those of watermelon silver mottle tospovirus (WSMV) from Taiwan and peanut bud necrosis tospovirus (PBNV) from India, the two definitive species of serogroup IV. Amino acid sequence similarity was 84% and 82% with WSMV and PBNV, respectively. On the basis of the sequence divergence and the previously determined host range differences, the watermelon tospovirus, designated as watermelon bud necrosis tospovirus, should be considered as a distinct species belonging to serogroup IV.

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Natural infection of tospoviruses on three cucurbitaceous (Cucumis sativus, cucumber; Luffa acutangula, ridge gourd; Citrullus lanatus, watermelon) and three fabaceous (Vigna unguiculata, cowpea; Phaseolus vulgaris, French bean; Dolichos lablab, sem) vegetable crops in India was identified on the basis of nucleocapsid protein (NP) gene characteristics. The complete NP gene of the cowpea isolate from Kerala and the sem isolate from Tamil Nadu was 831 nucleotides long, encoding a protein of 276 amino acids. For other Tospovirus isolates from cucumber, French bean, ridge gourd and watermelon, the partial NP gene (291 nt) was sequenced. Comparative NP gene sequence analyses revealed that fabaceous isolates shared maximum identity both at the nucleotide (92-97%) and amino acid (93-97%) levels with the corresponding region of Groundnut bud necrosis virus (GBNV), whereas cucurbitaceous isolates shared maximum identity both at nucleotide (93-99%) and amino acid (95-98%) levels with the corresponding region of Watermelon bud necrosis virus (WBNV), results suggesting that the Tospovirus isolates infecting fabaceous hosts should be regarded as strain of GBNV, whereas those infecting cucurbitaceous hosts as a strain of WBNV. Nucleocapsid protein gene was conserved both in GBNV and WBNV isolates originating from different hosts and locations.

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