MSc

Black gram (Vigna mungo (L.) Hepper) is one of the important pulse crop in
India and an excellent source of good quality protein. Yellow mosaic disease (YMD),
caused by a whitefly transmitted geminivirus is the major constraint for black gram
cultivation. Yellow mosaic disease of pulses is extensively studied from different
parts of the country. But no study has been conducted in Kerala. Hence, the present
study was undertaken to characterize the virus causing yellow mosaic disease in black
gram in Kerala and to evolve suitable strategies for its management.
Purposive sampling surveys were conducted in black gram growing areas of
Palakkad and Malappuram districts, covering 13 fields at nine locations to study the
incidence and symptomatology of yellow mosaic disease. The disease incidence in
the fields varied from 12 to 100 per cent.
Common symptoms observed in the field were typical yellow mosaic,
puckering and cupping of leaves, distortion of leaf lamina and drastic reduction in
size of younger leaves. Irregular whitish discolouration of leaves which turned papery
white on maturity was also observed in some fields. Complete yellowing of leaves
along with brown discoloration between the veins and vein banding symptoms which
were not reported by earlier workers were also observed in the field.
Electron microscopic studies revealed the presence of geminate particles of
15-18 x 30 nm size in infected black gram samples suggesting the association of a
geminivirus with the disease. Whitefly transmission of the virus to healthy black
gram plants was attempted and 80 per cent transmission was achieved.
PCR amplification using virus specific primers revealed the presence of
Mungbean yellow mosaic virus (MYMV) and Horsegram yellow mosaic virus
(HYMV) in the samples. MYMV was detected in infected samples from nine fields
out of the 13 fields surveyed. HYMV was detected in six fields, which include five
fields in which MYMV was also detected revealing the occurrence of mixed infection
in the field. Five representative isolates were sequenced at Agrigenome Labs,
Kakkanad, Ernakulam. In silico analysis of these sequences revealed that the coat
protein region of the isolates showed more than 90 per cent homology with MYMV
isolates. This confirmed the presence of MYMV as the major virus in yellow mosaic
disease of black gram in Kerala. Phylogenetic analysis revealed that the isolates from
the present study are more closely related to MYMV isolates from southern parts of
India and distantly related to Mungbean yellow mosaic India virus (MYMIV)
isolates, which were reported from northern parts of India.
Host range studies conducted in insect proof cages under glass house
condition showed that the virus could be transmitted through whiteflies only to horse
gram. Symptoms were observed in horse gram and Synedrella nodiflora, a
predominant weed found in the field 20 -25 days after inoculation. But the presence
of virus was confirmed by PCR only in symptomatic horse gram and not in
Synedrella nodiflora.
A field experiment was conducted during Rabi 2017-18 at RARS, Pattambi
and evaluation of effectiveness of botanicals, biocontrol agents and other organic
products revealed that application of Pseudomonas fluorescens as seed treatment @
10g/kg seed and foliar sprays @ 10g/l at fortnightly intervals starting from 15 days
after sowing or foliar sprays of 10 per cent aqueous extract of leaves of Bougainvillea
spectabilis or roots of Boerhaavia diffusa at fortnightly intervals starting from 15
days after sowing are effective in reducing the yellow mosaic disease.
The present study reveals that MYMV is the virus associated with the YMD
of black gram in Kerala and it can be effectively managed by prophylactic application
of Pseudomonas fluorescens, 10 per cent leaf extract of Bougainvillea spectabilis or
10% root extract of Boerhaavia diffusa. This is the first report on identification of
MYMV associated with yellow mosaic disease of black gram in Kerala.