PhD

A survey revealed that sweet potato feathery mottle is widely prevalent
in the farmers' fields of sweet potato at Thiruvananthapuram district of Kerala
and the types and intensity of symptoms varied between varieties. Chlorotic
leaf spot, pink spot, ring spot and feathering are the major symptoms observed.
No vectors were present and no tuber symptoms were found except tuber
constrictions in one variety, Vavvathooki. In symptomatology studies also;
similar types of symptoms with various combinations were observed. Initial
symptom was found after 15 days and the highest intensity between 30 to 60
days after planting in pots. The intensity of symptoms got reduced as the
intensity of shade was increased. The virus was readily transmitted through
single node, double node and triple node vine cuttings, tubers and three types of
graftings, viz., approach grafting, side grafting and wedge grafting. It was non-
persistently transmitted by Aphis craccivora, A. gossypii and through sap from
infected 1. nil to 1. batatas. However, sap transmission from 1. batatas to
1. batatas was not achieved. There was no transmission either through seed or
through Pentalonia nigronervosa and Bemisia tabaci. Photosynthesis was
reduced only in the susceptible variety Sree Rethna due to leaf infection but
respiration and transpiration were not affected in both the susceptible and
tolerant varieties. The infection caused a reduction in the content of total
carbohydrates, starch, total chlorophyll, chlorophyll'a' and chlorophyll'b' and
polyphenoloxidase activity in leaves of susceptible varieties, Sree Rethna and
Vavvathooki. In leaves, total sugars, total phenolics, protein, peroxidase and
phenylalanine ammonia lyase activities were not affected by the infection.





There was no significant change in number of vines, length of vine, number of
leaves, leaf area, total biomass and drymatter caused by SPFMV - infection
except an increase in internode length. There was no colour change in tubers.
Tubers of Vavvathooki plants infected with SPFMV showed constrictions
,
increase in length and reduction in girth. In tubers, quantities of total
carbohydrates, starch and drymatter were reduced while protein and sugars were
not affected due to infection. The virus was purified and infectivity was tested.
The antiserum produced was confirmed by agar double diffusion test for the
presence of antibodies. Detection of SPFMV in samples was done through
DAC-ELISA and NCM-ELISA. The dilution end point (DEP) of SPFMV was
found to be between 10-3 and 10-4, thermal inactivation point (TIP) between 60 and
65°C and longevity in vitro between 6 and 8 h at room temperature (28± 4°C) and
10 and 12 h at 8°C. The petiole sample was ideal in electron microscopy for quick
detection of SPFMV. The particles were long flexuous rods measuring
approximately 748 nm in length. In host range studies, among 25 plant species
from nine families, eight species, three from Convolvulaceae, four from Solanaceae
and one weed species fro!ll Asteraceae were found to act as collateral hosts of the
virus. Hot water treatment between 34 to 42°C for 5, 10 and 15 min and dry heat
incubation of cuttings at 37° and 39°C for different periods were not effective in
inhibiting the virus. In the induction of resistance, the chemical barium chloride
(0.015%) showed highest per cent inhibition of infection in 1. nil plants followed by
manganese chloride and the least by salicylic acid. Through meristem culture 85
and 96 per cent of virus free sweet potato plants were produced from the two
varieties, Sree Rethna and Vavvathooki. Storage of vine cuttings for disease
management was unsuccessful.