MSc

Watermelon, Citrullus lanatus (Thunb.), is an annual trailing creeper
belonging to the family Cucurbitaceae. It occupies a pivotal position among fruit
vegetables. Among the diseases affecting watermelon, those caused by viruses are
difficult to control and can be very destructive.

Recently, bud necrosis, a Tospovirus disease, has emerged as a serious
problem of watermelon cultivation in Kerala. Not much work on the various aspects
of the disease has been carried out. Further, little is known on disease incidence,
severity and mode of transmission of the disease which are crucial for evolving
appropriate management practices. Hence, the present project was undertaken to
study the symptomatology, transmission, physical properties, host range and
serological reaction of Watermelon bud necrosis virus (WBNV).
Survey undertaken on the incidence and severity of the disease in major watermelon growing locations of Thrissur, Palakkad, Malappuram, Kasaragod and
Kollam districts revealed the variations, based on the genotype, season and location.
Under natural conditions, symptoms observed were curling, mottling,
narrowing of leaf lamina, stunting, necrotic streaks on stem, unopening of flower
buds and their necrosis, necrotic rings and malformation of fruits. Under artificial
inoculation, symptom initiated as curling, crinkling and mottling of leaves, which
later spread to young leaves and resulted in brittleness.


The virus was isolated from symptomatic watermelon plants by mechanical
transmission through a local lesion host Amaranthus viridis using potassium
phosphate buffer pH 7.0.

Transmission studies proved that WBNV transmitted both by sap and vector. Citrate
phosphate buffer 0.1M (pH 7.2) showed the maximum transmission (72.22 %) with the minimum incubation period (8 days) and was used for further studies. The insect
vector of WBNV was identified as Thrips palmi Karny. Studies on virus vector
relationships showed that the minimum acquisition access period and inoculation
access period were 24 h and 48 h respectively.

Physical properties of virus i.e., dilution end point, thermal inactivation point
and longevity in in vitro were investigated and it was found that the virus was
inactivated at dilutions between 10-2 and 10-3 and at temperatures between 45
and500C. Longevity in in vitro was recorded as four h at room temperature (28± 2) and
24 h under refrigerated condition (80 C).
Host range studies revealed that ridge gourd, bottle gourd, salad cucumber,
bitter gourd, cowpea, groundnut and gomphrena could serve as collateral hosts of the
virus. Serological tests using DAC – ELISA of WBNV revealed close relation of the
virus with Peanut bud necrosis virus.
Among the 22 genotypes of watermelon screened for response to the virus
none of them showed resistance. However, WMH531 and WMH 12031 recorded the
lowest incidence of 10 and 8.33 per cent respectively, while the highest (96 per cent)
was in NS 295 hybrid.