MSc

The research work entitled ‘Prevalence of sesamum phyllody in Onattukara
tract and evaluation of fungal root endophyte Piriformospora indica for its
management’ was conducted during 2019-21 at Department of Plant Pathology,
College of Agriculture, Vellayani and Onattukara Regional Agricultural Research
Station with the objectives to study the symptomatology, molecular detection and
characterization of phytoplasma inciting sesamum phyllody disease in AEU 3
(Onattukara tract); and evaluation of fungal root endophyte P. indica for its
management.
Phytoplasma infected sesamum samples were collected from D and F blocks
of Onattukara Regional Agricultural Research Station and Karthikapally.
Karthikapally recorded highest disease incidence (39.44 per cent) and vulnerability
index (23.75). Chocolate weed, Melochia corchorifolia, was found to be exhibiting
symptoms of shoot proliferation. Hoppers collected from the infected fields were
identified as Orosius albicintus, Hishimonas phycitis and Nephotettix sp.
Disease symptoms were observed at the stage of flowering of sesamum plants
in all the sampled locations. The associated symptoms were reduction in internodal
length of stem, axillary bud proliferation, thickening of the floral veins, phyllody and
floral proliferation. Microtome sections of infected and healthy leaf, stem of sesamum
stained with 4,6-diamidino-2-phenylindole (DAPI) stain, and observed under
fluorescence microscope emitted diffuse fluorescence from the infected tissues, which
was brighter than the one from the parenchymal cells indicating the presence of
phytoplasma in the infected tissues.
Studies on variations in the level of gibberellic acid (GA) and indole-3-acetic
acid (IAA) in phyllody infected and healthy sesamum was undertaken. GA content
was increased by 2.25 times and 10.46 times, and IAA content was decreased by 1.25
times and 1.97 times in leaves and flowers of infected samples compared to the
healthy samples.
Molecular characterization of sesamum phyllody was performed with leaf
samples collected from ORARS lowland, ORARS upland, Vellayani and
Karthikapally. Amplicons of 1.4kb was obtained by amplifying with universal primers
P1/P6 for detection of phytoplasma. The sequences obtained were subjected to
BLAST analysis and the 16S rDNA gene sequence showed that all the isolates shared
more than 99 per cent similarity with that of the ‘Candidatus phytoplasma
aurantifolia’ strains in GenBank data base. In the phylogenetic tree constructed, the
sesamum phyllody phytoplasma under study clustered with the 16SrII group
(Candidatus Phytoplasma aurantifolia) phytoplasmas causing sesamum phyllody in
various regions. The virtual RFLP pattern generated by iPhyClassifier, derived from
16S rDNA fragment was found to be identical to the reference pattern of 16Sr group
II, subgroup D (GenBank accession: Y10097). Based on the results obtained from
sequence analysis and virtual RFLP pattern, the phytoplasma associated with
sesamum phyllody was identified as ‘'Candidatus Phytoplasma aurantifolia”-related
strain belonging to subgroup 16SrII-D.
P. indica obtained from Department of Plant Pathology, College of
Agriculture, Vellayani was mass multiplied in sterilized coir pith: FYM mixture (1:1)
amended with 2 per cent gram flour and sesamum seeds were sown. Colonization was
observed seven days after germination. Wedge grafting was standardized in sesamum
at 30 days after germination. Pot culture experiment was conducted to evaluate the
effect of P. indica against phytoplasma causing sesamum phyllody, by grafting the
colonized and non-colonized plants with infected scion. P. indica colonization could
significantly reduce the incidence and severity of infection. After 30 and 45 days of
grafting, an incidence of 20 and 60 per cent, and severity of 5 and 50 were recorded in
the colonized plants grafted with infected scion, whereas an incidence of 60 and 80
per cent and severity of 45 and 75 were recorded in non-colonized plants grafted with
infected scion. In colonized plants, enhanced shoot and root length at 30 and 55 days
after germination were recorded and also earliness in flowering compared to noncolonized plants.
Hence the associated symptoms of phytoplasma infection in sesamum are
virescence, thickening of floral veins, phyllody and floral proliferation. The study
revealed the association of Candidatus phytoplasma aurantifolia group with sesamum
phyllody prevalent in Onattukara tract. The evaluation of beneficial fungal root
endophyte P. indica against phytoplasma revealed delayed expression of symptoms in
the colonized plants.