Abstract:
Investigations were undertaken on the virus causing severe mosaic on
cowpea (Vigna unguiculata (L.) Walp) in Kerala. The characteristic symptoms
appeared as vein clearing, light and dark green mottling, severe mosaic, dark
green vein banding, blistering, distortion and reduction in leaf size. The virus was
mechanically transmitted through sap extracted in 0.01 M phosphate buffer
(pH 7.0). The virus was efficiently transmitted by the aphid vector, Aphis craccivora.
Seed transmission of eleven per cent was recorded in the variety Sharika. Thermal
inactivation point was recorded at a range of 60 - 65° C, dilution end point at a
range of 10-3 - 10-4 and longevity in vitro for four hours at room temperature
(28 ± 4° C) and six hours under refrigerated condition (8° C). A. craccivora could
efficiently transmit the virus with an acquisition access of ten minutes and
inoculation access of one minute. Pre-acquisition starvation increased the rate of
transmission while post-acquisition starvation decreased the rate. A single aphid
was capable of transmitting the virus. The virus causing severe mosaic was
identified as blackeye cowpea mosaic virus by ELISA. The virus could also be detected
by Ouchterlony immunodiffusion test. Electron microscopic studies revealed the
presence of flexuous, filamentous particles of 750 nm in length. Two varieties
Co-6 and Cc-Selection were grouped as no symptom producing among 65
genotypes screened for resistance. Fifty three F2 progenies of the cross Sharika
and Co-6 and twenty five F2 progenies of the cross Co-Selection and Sharika were
long poded and resistant. Biochemical changes indicated a lower carbohydrate
content in resistant compared to susceptible. Chlorophyll content decreased in the
susceptible variety due to virus infection. Increase in protein was observed in both
resistant and susceptible. The phenol content did not show variation between the
varieties.
Peroxidase, polyphenol oxidase and phenylalanine ammonia-lyase
activities increased in the resistant variety. Bioassay of chemicals and neem oil on
local lesion host (c. amaranticolor) indicated a per cent inhibition of 68.92 by
neem oil in pre-inoculation application and 65.45 per cent inhibition by manganese
chloride in post-inoculation application. On cowpea plants, pre-inoculation
application of neem oil (ten per cent) concentration was found to be effective in
reducing the symptoms due to viral infection.
