Watermelon bud necrosis orthotospovirus - an emerging constraint in the Indian subcontinent: an overview.
Watermelon bud necrosis orthotospovirus (WBNV), is a distinct virus species in the genus Orthotospovirus, family Tospoviridae and order Bunyavirales. Orthotospovirus is the only genus in the family whose members infect plants, while viruses in other genera infect invertebrates. WBNV is prevalent in most of the watermelon growing Indian states and therefore, its prevalence in the Indian subcontinent cannot be ignored. Quasi-spherical enveloped virions are 80-110 nm in size. Virion composed of genomic RNAs with nucleoprotein, viral polymerase, and a lipid envelope with glycoprotein (GN/Gc) spikes. WBNV shares common genomic features with other orthotospoviruses. The tri-segmented genome is referred to as the small (S) (3.4 kb), the medium (M) (4.7 kb), and the large (L) (8.9 kb) RNAs. S RNA uses ambisense coding strategy and codes for non-structural (NSs) protein in the viral (v) sense and nucleocapsid (N) protein in the viral complementary (vc) sense. Similarly, M RNA uses ambisense coding strategy and codes for a precursor for the GN/GC in vc sense and movement (NSm) protein in v sense. L RNA is of negative polarity and codes for the RNA-dependent RNA polymerase (RdRp). WBNV is known to be transmitted by Thrips palmi (Order: Thysanoptera; Family: Thripidae; melon thrips) in a persistent propagative manner. WBNV is sap transmitted to various experimental hosts belonging to several families viz., Amaranthaceae, Asteraceae, Chenopodiaceae, Cucurbitaceae, Fabaceae, Pedaliaceae, and Solanaceae. WBNV has limited natural host range, predominantly confined to bitter gourd (Memordica charantia), chrysanthemum (Chrysanthemum indicum), chilli (Capsicum annum), cucumber (Cucumis sativus), muskmelon (Cucumis melo), pumpkin (Cucumis sativus), ridge gourd (Luffa acutangula), tomato (Solanum lycopersicum), and watermelon (Citrullus lanatus). Besides these main hosts, Abutilon indicum, Amaranthus spp., Euphorbia heterophylla, Malvastrum coromandelium, and Parthenium hysterophorus serve as green-bridge for virus perpetuation. WBNV symptoms include mottling, yellowing, necrotic streaks on vines, shortened internodes, stunted plant growth, severe stem and bud necrosis and dieback of the buds. Late season infection results in deformed fruits with necrotic local lesions. In India, WBNV resistant watermelon varieties are not available. A wild relative of watermelon, Citrullus colosynthis, has recently been identified as a resistant source, which is used in the breeding program as part of integrated disease management (IDM) strategies. IDM strategies include clean crop cultivation, border cropping, intercropping, use of plastic mulches, crop rotation, and control of thrips can alleviate the effect of WBNV in the Indian subcontinent. Recently, non-conventional management option of development of putative transgenic resistance has been worked out and found WBNV N gene is effective in conferring resistance in watermelon.