MSc

Rice is the staple food crop for the people of Kerala and traditionally, the
cultivation of rice has occupied pride of place in the agrarian economy of the state. After
the flood in 2018, wide spread occurrence of bacterial blight and sheath blight was
noticed in Kerala. Compared to previous years, the incidence of sheath blight was high
in 2019 also. In this context, the presentstudy was undertaken to isolate and characterize
Rhizoctonia spp. associated with sheath diseases of rice and to evaluate rice germplasm
accessions for finding resistance sources.
Purposive sampling surveys were carried out in AEU’s 10, 22 and 23 of Palakkad
district, AEU’s 6 and 10 of Thrissur district and AEU 6 of Malappuram district of Kerala
covering 24 locations. Survey revealed widespread occurrence of sheath diseases of rice
with varying symptoms. The disease severity and incidence also varied widely. In
surveyed locations, the predominant varieties Jyothi and Uma were affected with sheath
diseases. Disease severity ranged from 35.78 to 90.25 per cent and incidence ranged
from 50.99 to 84.23 per cent in Palakkad district whereas in Thrissur district, severity
ranged from 55.10 to 77.18 per cent and incidence ranged from 43.40 to 55.35 per cent.
In Malappuram district, severity of 93.04 per cent and incidence of 78.31 per cent were
recorded. Different symptoms observed in surveyed locations include small, round, oval
to elliptical spots having greenish grey center and pale brown to blackish brown wavy
margins. Irregular lesions with water soaked greenish grey center with pale brown to
brown margin and irregular patches with pale brown to straw coloured centers
surrounded by brown to black margins were also observed. Another type of symptom
seen was blackish spots of varying size and shapes and black discolouration on leaf
sheaths at the base of the plant. Pathogen was isolated from all the 50 samples and
pathogenicity was proved on susceptible rice variety Jyothi.
Cultural characters like colony texture, colony colour, zonation, days taken for
formation of sclerotia and growth rate were studied for all pathogen isolates. Colony
colour recorded were pale brown, white, whitish brown and creamy brown. Cottony,
fluffy and raised thread type of colony textures were recorded. Four out of the fifty
isolates formed zonation when cultured on PDA media. Isolates formed sclerotia on
PDA within four to seven days. Thirteen isolates did not form sclerotia.
Morphological characters such as colour and septation of hyphae, shape, colour,
size of sclerotia, number of sclerotia formed per plate, number of sclerotia formed on
under surface of lid, pattern of sclerotia formation and sclerotial texture were studied.
Hyphal colour were initially white later turned into pale brown, whitish brown, creamy
brown or remained whitish. For all the isolates, hyphae was septate with right angled
branching. Spherical, tiny spherical and irregular shaped sclerotia were produced by the
pathogen isolates. Sclerotial colour ranged from brown, pale brown and dark brown.
Sclerotial size ranged from 0.57 to 1.70 mm. Number of sclerotia formed per plate
ranged from two to 138. Sclerotia were formed in different patterns. In some of the
isolates, sclerotia were scattered. In some, it formed on upper lid, in a few they were
concentrated towards centre or periphery or on the rim of Petri plate. Sclerotia of
different isolates were of varying textures such as smooth and rough texture with
secretions on it. Based on these characters, all isolates of Rhizoctonia spp. were
tentatively identified as Rhizoctonia solani.
Molecular characterization was done by using species specific primers. PCR
amplification with primer pairs GMRS-3 and ITS1, GMRS-4 and ITS1 which are
specific to R. solani produced 550 and 650 bp amplified product respectively for all the
isolates confirming that all of them were R. solani. The ITS sequencing results also
confirmed the identity of pathogen isolates Sb1, Sb13, Sb16, Sb21, Sb30, Sb34, Sb36,
Sb38, Sb39 and Sb45 as Rhizoctonia solani. Genetic variability analysis of the R. solani
isolates was carried out by Rep-PCR by using BOXA1R and ERIC2 primers. Fifteen
distinct bands were generated by BOXA1R primer and 12 distinct bands were generated
by ERIC2 primer. Dendrogram was constructed by using NtSys cluster analysis
software which revealed that variability exists among the isolates of R. solani even
within the same geographical location.
With the aim of identifying sources of resistance against sheath blight disease,
50 rice germplasm accessions from RARS, Pattambi were screened. Forty seven
accessions were highly susceptible with disease severity more than 66 per cent (score
9), two accessions were susceptible with severity of 46.31 and 64.65 per cent (score 7)
and one accession was moderately susceptible with disease severity of 45.23 per cent
(score 5). Even though, none of the genotypes could be located with resistance to sheath
blight, the information generated from this study would be useful for further utilization
of germplasm for breeding purpose. The reaction of these accessions will be added to
the database so that while selecting any of these accessions, the susceptibility of these
accessions to sheath blight can be taken into consideration.