MSc

The present study was undertaken in an endeavor to gain some understanding
regarding the viral diseases affecting chilli and devising suitable management
strategies to contain them. A purposive sampling survey conducted involving 3
districts of Kerala, namely Palakkad, Thrissur and Malappuram covering over 15
locations and 44 samples revealed symptoms descriptive of viral infection like
curling, puckering, cupping and yellowing of leaves, dwarfing of plants and low fruit
set in affected plants. To identify the etiological agent(s) associated with the
symptoms, transmission studies were carried out. Whitefly transmission and graft
transmission gave positive results whereas none of the samples that were sap
transmitted exhibited symptoms indicating the presence of a geminivirus. Detection
methods like Transmission Electron Microscopy (TEM) and Enzyme Linked
Immunosorbant Assay (ELISA) were also carried out. Geminate particles were
observed but no mosaic causing virus was detected in EM. ELISA performed on
samples with mosaic like symptoms for the presence of potyvirus and CMV did not
detect the presence of suspected virus in it. Hence, it was concluded that leaf curl
disease was the only major disease affecting the crop in the area.
Characterization of the virus (s) responsible for the leaf curl disease was
attempted. DNA was isolated from seven representative samples after which PCR
(Polymerase Chain Reaction) was performed with two sets of primers, degenerate
(DENG) primer and a specific primer. The PCR amplified products of four
representative isolates were sequenced and analysed for homology with sequences
available in NCBI using BLAST software. Three of the isolates showed maximum
nucleotide identity to the Pepper leaf curl Bangladesh virus isolate
India/Coimbatore/Chilli/2008 segment DNA-A, complete sequence (HM007096.1)
and the other isolate showed maximum identity with Chilli leaf curl virus-India
isolate Raichur segment DNA-A, complete sequence (MK161454.1).
A phylogenetic tree was constructed combining different geminiviruses
reported from different crops to study the sequence similarity existing amongst them.
The amino acid sequence of the coat protein gene of the isolate PKD-1 was predicted
using the ExPASy translation tool available online.
With the aim of identifying source (s) of resistance against chilli leaf curl
disease in Kerala, two sets of screening experiments were carried out. In the initial
field screening of 30 accessions, Violet Mulak, a local accession remained
symptomless whereas Arka Lohit (IIHR) and Phule Jyothi (MPKVV) remained highly
resistant. The molecular confirmation of the incidence of Chilli leaf curl virus
(ChiLCV) in the field was done by performing PCR using primers specific to the CP
gene region of the concerned virus. A pot experiment was set up to once again score
the disease reaction of 48 accessions including those used in field screening along
with a few more accessions. In this experiment, Phule Jyothi remained symptomless
and Violet mulak turned out highly resistant whereas Arka Lohit remained HR.
Twentysix random infected accessions were confirmed for the presence of ChiLCV in
them.
The three accessions that were adjudged highly resistant following the
conclusions of the abovesaid two screening experiments were subjected to graft
transmission. Fifty days old field exposed plants of the resistant sources were used as
rootstocks in wedge grafting with scions from known susceptible variety. At 60 days
of grafting, rootstocks and scions failed to amplify the primer indicating the absence
of virus in them. From all these experiments, what could be inferred is that these
accessions possessed true resistance against the virus up to 50 days after transplanting
in the field.
A management study to evaluate the efficacy of certain plant extracts and
biocontrol agents in containing chilli leaf curl disease was undertaken the results of
which indicate spraying the leaf extract of Azadirachta indica (10 %) or Mirabilis
jalapa (10 %) as the best treatment. The only biocontrol agent with an effect on par
with the best treatments was Bacillus subtilis applied as seed treatment (10g kg-1 seed)
and foliar spray (20g l-1). All the four plant extracts did have a reducing effect on the
disease severity with the least mean recorded in A. indica (10 %).