First report of the natural occurrence of Tobacco streak virus on blackgram (Vigna mungo)

D. Ladhalakshmi¹*, M. Ramiah¹, T. Ganapathy¹, M. Krishna Reddy³, Salah Eddin Khabbaz¹, Merin Babu¹ and A. Kamalakannan²

¹ Department of Plant Pathology, Tamil Nadu Agricultural University, Coimbatore, India
² Agricultural Research Station, Tamil Nadu Agricultural University, Bhavanisagar, India
³ Senior Scientist, Indian Institute of Horticultural Research, Hassargutta, Bangalore, India

Accepted for publication 02/09/05

Blackgram (Vigna mungo; family Fabaceae) is an important pulse crop in India, Nepal, Bangaladesh and Myanmar. During kharif season (June-September 2002), a new disease was observed in the majority of the blackgram growing areas of India. Symptoms of the disease consisted of brown necrotic lesions on young leaves (Fig. 1A), with brown streaks on petioles and stems (Fig. 1B). In severe cases infected plants were found dead (Fig. 1C). Mechanical inoculation of plant sap from infected plants onto various plant species: Vigna unguiculata cv. 152 (Fabaceae), Nicotiana tabaccum (Solanaceae) (Fig. 2), Chenopodium amaranticolor (Chenopodiaceae) and Gomphrena globosa (Amaranthaceae), produced necrotic lesions on leaves and streaks on stems. Alternate hosts may also play role in disease epidemiology; since TSV infects many widely distributed weeds. For example, Parthenium hysterophorus probably plays a major role in the spread of TSV in peanut (Arachis hypogaea) (Prasada Rao et al., 2003).

Figure 1: Symptoms associated with natural occurrence of Tobacco streak virus on blackgram: left, brown necrotic lesions on the young leaves; middle, brown streaks on the petiole and stem; right, petiole and stem necrosis of TSV infected plants.

Infected leaf samples of blackgram tested positive for TSV by DAC-ELISA (Bhat et al., 2001) using a specific polyclonal antibody (a gift from G. Cook, Plant Protection Research Institute, South Africa). Electron microscopic observations of negatively-stained preparations of purified virus from infected blackgram leaves revealed spheres with a diameter of 27 nm; consistent with TSV infection. RT-PCR tests of tissue from diseased blackgram plants using primers specific for the coat protein gene of TSV (Bhat et al., 2002) resulted in an amplicon of the expected size: approximately 700bp (Fig. 3). TSV infects a wide range of hosts in India (Prasada Rao et al., 2003). This is the first report of the natural occurrence of TSV on Vigna mungo.

Figure 2: Necrotic rings on the leaf of Nicotiana tabaccum	Figure 3: Agarose gel electrophoresis of RT-PCR products: lane 1, 100bp DNA ladder; lane 3, healthy; lane 4, infected urdbean sample

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The British Society for Plant Pathology