MSc

The study entitled “Etiology and immuno-molecular detection of the
virus causing mosaic disease in snake gourd” was carried out at College of
Agriculture,
Vellayani during 2021-2023 with the objectives of
symptomatology, maintenance of virus in systemic and local lesion host, host
range studies, seed transmission, aphid transmission, immunological diagnosis,
molecular diagnosis and host pathogen interaction of the viruses causing
mosaic disease in snake gourd (Trichosanthes anguina).
Mosaic diseased snake gourd samples were collected from three taluks
of Thiruvananthapuram district (Thiruvananthapuram, Neyyattinkara and
Varkala) of Kerala during January-March and July-September of 2021-23. The
characteristic symptoms of snake gourd mosaic disease were mosaic, mosaic
mottling, vein banding, blistering, distortion, puckering on leaves and
deformed fruits. The disease incidence in majority of the fields was found to be
over 25.0 per cent and disease severity progressed with age of the crop. In the
surveyed locations, the highest disease incidence of 94.0 to 100 per cent and
vulnerability index of 72.9 to 82.7 were observed in commercial variety
Kaumudi from fields of College of Agriculture, Vellayani. In snake gourd
variety Baby, disease incidence of 28.5 per cent and vulnerability index of 46.6
were observed. Similar symptoms of mosaic were observed on weeds like
Hemidesmus indicus (F: Apocynaceae) and Synedrella nodiflora (F:
Asteraceae) in snake gourd field. The snake gourd mosaic viruses were sap
transmissible and maintained in systemic host viz., snake gourd varieties
Kaumudi and Baby and also in local lesion hosts viz., Chenopodium
amaranticolor and Nicotiana tabacum var. Samsun. Local lesion assay
revealed that viruses associated with snake gourd mosaic disease were
Potyviruses and Cucumoviruses.
Based on host range studies, virus was mechanically transmissible to
plants of family Cucurbitaceae viz., pumpkin, bitter gourd, cucumber, ridge
gourd, bottle gourd, water melon and family Solanaceae viz., Nicotiana
glutinosa and Nicotiana tabacum var. Samsun. However, no symptoms were
produced in tomato, chilli, brinjal and papaya. Host range studies identified
viruses associated with the snake gourd mosaic disease as Papaya ring spot
virus (PRSV) Type W and Cucumber mosaic virus (CMV). No seed
transmission was recorded in snake gourd plants raised from seeds produced by
snake gourd mosaic viruses infected plant. Aphid transmission of the virus of
30.0 per cent was observed with Aphis craccivora and 50.0 per cent was
observed with A. gossypii.
The viruses causing snake gourd mosaic disease were immunologically
detected using Double antibody sandwich-Enzyme linked immunosorbent
assay (DAS-ELISA) and Direct antigen coating-ELISA (DAC-ELISA) with
polyclonal antibodies specific to PRSV and CMV respectively. Dot
immunobinding assay (DIBA) detected CMV from snake gourd mosaic
diseased samples. Reverse transcriptase-Polymerase chain reaction (RT-PCR)
was carried out and amplicons of sizes 1200 bp and 400 bp were obtained with
primers specific to coat protein of PRSV and 2a protein of CMV respectively.
Comparative nucleotide sequence alignment of the isolates revealed an 86.5
and 93.8 per cent homology with PRSV and CMV isolates from Tamil Nadu
respectively. Comparative amino acid sequence alignment of the isolates
revealed an 88.9 and 97.8 per cent homology with coat protein of PRSV-W and
2a protein of CMV respectively. The sequences were deposited in NCBI
GenBank with accession numbers viz., OR601008 and OR601007.
Phylogenetic tree was constructed and PRSV isolate grouped in the same clade
along with other reported PRSV type-W isolates, thus the isolate was identified
as PRSV Type-W. CMV isolate from snake gourd grouped in the same clade
along with other CMV isolates from snake gourd and Musa sp.
Virus inoculated snake gourd plants showed a significant reduction in
total chlorophyll content (0.61 mg g-1) and increase in protein content (13.79
mg g-1) at 5 weeks after transmission. The activities of defense related enzymes
(peroxidase, polyphenol oxidase, phenylalanine ammonialyase, superoxide
dismutase and catalase) were higher in inoculated plants compared to healthy
plants.
Thus, the present study revealed that snake gourd mosaic disease is
caused by the combined infection of PRSV-W and CMV in the surveyed
locations of Thiruvananthapuram district. Synedrella nodiflora was identified
as the suitable weed host and viruses were transmitted through Aphis gossypii
and Aphis craccivora.