Outbreak of tobacco streak virus causing necrosis of cucumber (Cucumis sativus) and gherkin (Cucumis anguria) in India

M Krishnareddy, Devaraj, Lakshmi Raman, Salil Jalali, DK Samuel, Division of Plant Pathology, Indian Institute of Horticultural Research, Hessaraghatta Lake PO, Bangalore-560089, India. Plant Dis 2003; 87: 1264; published online as D-2003-0804-03N, 2003. Accepted for publication 6 Jun 2003.

Cucumber (_Cucumis sativus_ L.) and gherkin (_Cucumis anguria_ L.) are important cucurbitaceous vegetables grown in India for slicing and pickling. During the 2000 to 2002 rainy season and summer, a new viral disease, causing yield losses of 31 to 75 per cent in Bangalore, Bellary, Davanagiree, and Tumkur districts of Karnataka State, infected cucumber and gherkin.

Symptoms were tip necrosis characterized by necrotic lesions on leaves, and a general leaf and stem necrosis extending to mid veins, petioles, flower buds, and tip, eventually resulting in die-back of vines.

Tissue extracts from symptomatic leaves of cucumber and gherkin were mechanically inoculated on several herbaceous indicator plants (cowpea, cucumber, pepper, Zinnia, watermelon, _Chenopodium amaranticolor_, sunflower, _Nicotiana glutinosa_, _N. tabacum_, and _Gomphrena globosa_). On most hosts, symptoms of chlorotic or necrotic lesions followed by mottle or systemic necrosis were observed.

Back-inoculation from the symptomatic indicator plants onto cucumber and gherkin resulted in symptoms typical of those observed in the field.

Electron microscopic examination of leaf-dip preparation and ultra-thin sections of virus infected plant samples showed the presence of isometric particles 25 to 28 nm in diameter. Similar types of particles were observed when infected samples were trapped in immunosorbent electron microscopy with polyclonal antibodies specific to tobacco streak ilarvirus (TSV) but not to watermelon silver mottle tospovirus (WSMoV).

Enzyme linked immunosorbent assay tests using leaf extracts of field-collected samples and sap-inoculated plants showed positive reaction to antibodies of TSV (1) but not to antibodies of cucumber mosaic cucumovirus, WSMoV, watermelon bud necrosis tospovirus, papaya ringspot potyvirus W strain, and zucchini yellow mosaic potyvirus.

Reverse transcription-polymerase chain reaction (RT-PCR) of RNA extracts of infected samples of field and inoculated symptomatic plants was done by using primers derived from TSV RNA3 specific for the coat protein (CP) region of TSV (2). A 800-bp specific DNA fragment was amplified from infected cucumber and gherkin but not from healthy control plants.

Sequence analysis of cloned PCR fragments revealed nucleotide identities of 99 per cent with TSV isolates from cotton, mungbean, sunnhemp, and sunflower (GenBank Accessions Nos. AF515824, AF515823, AF515825, and AY061929) and 88 per cent with TSV-WC (GenBank Accession No. X00435).

On the basis of host range, serological relationship, electron microscopy, and sequence analysis of the CP region, the virus was identified as a strain of TSV. To our knowledge, this is the first report of natural occurrence of TSV on cucumber and gherkin in India.

References:
(1). AI Bhat, et al. Arch Virol 2002; 147: 651.
(2). BJC Cornelissen, et al. Nucleic Acids Res 1984; 12: 2427.