MSc

The study entitled „Etiology of bud proliferation in vegetable cowpea‟ was
conducted at Department of Plant Pathology, College of Agriculture, Vellayani during
2019-2021 with an objective of studying the symptomatology, immuno-molecular
detection and characterization of incitant/(s) of bud proliferation in vegetable cowpea.
Bud proliferation disease of cowpea has been observed in different varieties in
different locations. Purposive sampling was carried out in different locations of
Thiruvananthapuram, Kollam, Alappuzha and Thrissur and collected symptoms were
used for further studies.
The characteristic symptom of the disease is the abnormal proliferation of bud
which showed an increase in number of buds up to 16-35. The proliferated buds also
showed pinkish- brown patches in it. Noticeable symptoms were also seen in the
leaves of the diseased plant. They were smaller, crinkled and unusually dark in colour.
The fasciation of stem has also been observed in the fields of Onattukara and Chavara.
The infected plants were stunted and completely sterile. The highest disease incidence
was found to be in the variety Bhagyalakshmy cultivated in Mannuthy (6.57 per cent).
The population of hoppers was observed in the field and the insects identified were
Exitanus sp., Balclutha sp., Nilaparvata lugens, Ptoleria sp., Nisia nervosa. The
weeds Phyllanthus, Neer-grampu and Jack bean were found to be showing similar
symptoms near the diseased cowpea fields.
The graft transmission was successful from cowpea to periwinkle with 40 per
cent efficiency. The leaves of the graft inoculated periwinkle plants showed severe
interveinal chlorosis and later on yellowing. Graft transmission was unsuccessful from
cowpea to cowpea. The transmission studies for viruses from cowpea to cowpea and
cowpea to Chenopodium revealed the absence of viruses.
The DAPI staining of diseased and healthy plants affirmed the presence of
phytoplasma in the diseased samples. Small fluorescent-coloured bodies were seen in
the stem and leaf of infected plants compared to healthy.
The hormonal analysis of the symptomatic plants compared to the healthy
ones showed significant difference. The GA content in diseased leaf and bud was
increased by 20.88 and 17.46 per cent respectively. The IAA content in diseased leaf
(older) and bud was increased by 61.55 and 46.52 per cent respectively.
The serological detection for viruses using monoclonal antibodies of CABMV
and BICMV and polyclonal antibodies of TSWV and WSMoV divulged the absence
of viruses in the diseased samples. The graft inoculated periwinkle plants also showed
no presence of viruses.
The molecular detection of phytoplasma with nested PCR was carried out. The
first primers P1/P7 amplified a 1.8kb fragment and the second set of primers;
R16F2n/R16R2 amplified a 1.2kb fragment, giving positive results. The final
amplified product was sequenced and by BLAST analysis it was found that the 16S
rDNA sequence shared 99.80 per cent similarity with that of the „Candidatus
Phytoplasma asteris‟ reference strain (GenBank accession: M30790). Hence the
phytoplasma under study is a „Candidatus Phytoplasma asteris‟-related strain. The
virtual RFLP pattern derived from the query 16S rDNA F2nR2 fragment was identical
(similarity coefficient 1.00) to the reference pattern of 16Sr group I, subgroup B
(GenBank accession: AP006628). The phytoplasma under study was found to be a
member of 16SrI-B.