MSc

The study entitled “Molecular characterization of coat protein gene of Tospovirus
isolate from vegetable cowpea and tomato” was carried out at College of Agriculture, Vellayani
during 2019-2021 with the objective of symptomatology, serological diagnosis and molecular
characterization of Tospovirus causing necrosis disease in vegetable cowpea [Vigna
unguiculata subsp. Sesquipedalis] and tomato [Lycopersicon esculentum Mill].
Plant samples showing the typical symptoms of Tospovirus were collected from two
taluks of Thiruvananthapuram (Thiruvananthapuram and Chirayinkeezhu) district of Kerala
during December to September, 2020-21. A higher incidence of Tospovirus (90%) on cowpea
was recorded in Kalliyoor, in a commercial cowpea variety Badari. 100% disease incidence by
Tospovirus was observed on tomato variety Vellayani Vijay at College of Agriculture,
Vellayani. The study on the disease incidence (DI) of infected crops in most of the fields
showed more than 25 per cent DI. From the study it was also observed that there is variation in
disease severity with age of crops. Weed plants Ageratum conyzoides (F: Asteraceae) and
Ludwigia perenis (F: Onagraceae) found to be associated with Tospovirus infection.
The virus was sap transmissible and was maintained in local lesion host, Chenopodium
amaranticolor and cowpea assay variety, Pusa Komal by mechanical inoculation using
potassium phosphate buffer. In the present study virus was mechanically inoculated to different
varieties of cowpea and tomato. Both mechanical as well as graft inoculation was tried in
tomato varieties. From the mechanical inoculation studies, the vegetable cowpea variety
Githika was tolerant compared to Lola and Vellayani Jyothika based on number of days taken
for symptom development and nature of symptoms and among the bush varieties, Co-6 found
to be tolerant. While in tomato, Variety Arka Rakshak found to be tolerant as compared to
Vellayani Vijay and Manuprabha.Successful graft transmission of Tospovirus observed in
tomato and it can be used as a reliable method for screening for disease resistance against
Tospovirus in tomato. No seed transmission was recorded in Tospovirus isolate of cowpea and
tomato and the seeds obtained from infected plants of tomato showed less germination
percentage.
The virus causing bud necrosis disease in cowpea and tomato was serologically
detected using ELISA and DIBA and found that Tospovirus isolate from both cowpea and
tomato showed close relationship with WSMoV. The virus was also detected using RT-PCR
and an amplicon of size 1000bp was obtained using primer specific to coat protein of GBNV
207
and WSMoV. The studies on cross infectivity indicated possible transmission of Tospovirus
isolate from cowpea to tomato and vice versa.
Activity of defense related enzymes such as peroxidase, polyphenol oxidase and
phenylalanine ammonia-lyase was more on inoculated plants than healthy control plants. The
activity was higher in tolerant variety of cowpea (Co-6) and tomato (Arka Rakshak).The
Protein and isozyme profile of both tolerant and susceptible cultivars of cowpea and tomato
showed higher protein and isozyme production in tolerant varieties pointing towards a positive
correlation of disease resistance to PR proteins and defense related enzymes.
Thus the present study revealed disease incidence due to tospovirus on cowpea and
tomato was more prevalent in Thiruvananthapuram (Thiruvananthapuram and Chirayinkeezhu
Taluks) district of Kerala. The serological and molecular characterization of cowpea Tospovirus
confirmed that virus belongs to serogroup IV which is Watermelon silver mottle
orthotospovirus. One of the tomato sample reacted positive to Tospovirus of serogroup I which
is Tomato spotted wilt virus indicating the presence of serogroup I and IV in tomato.
Association of Tospovirus in symptomatic weed plants seen in fields of both cowpea and
tomato can be positively correlated to the high disease severity in these areas. Mechanical
inoculation, enzyme activity studies, SDS protein and isozyme profiling indicated that
vegetable cowpea variety, Githika and tomato variety, Arka Rakshak have tolerance to
Tospovirus infection. Hence on the basis of present investigation, it can be concluded that
usage of tolerant genotypes like Githika in cowpea and Arka Rakshak in tomato along with
management of weeds like Ageratum and Ludwigia can reduce the disease severity to some
extent.