STUDIES ON RICE BLAST INCITED BY Pyricularia grisea (Cooke) Sacc. IN TELANGANA STATE
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Date
2020
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PROFESSOR JAYASHANKAR TELANGANA STATE AGRICULTURAL UNIVERSITY
Abstract
Rice blast caused by Pyricularia grisea (Cooke) Sacc. became one of the most
important disease in all rice growing areas of Telangana State because of its wide distribution
and destructiveness under favourable conditions. Keeping this in view detailed investigations
were carried out on symptomatology, cultural, morphological, pathological and molecular
variability of the pathogen.
Roving survey was conducted in major rice growing districts of Telangana State during
kharif, 2019 and collected a total of twelve blast infected rice samples showing typical
symptoms of the disease. The disease was characterized by the formation of spindle shaped
lesions on the leaves with greyish center and dark brown margins. Lesions on the neck were
identified as greyish brown spots which causes girdling of branches and over a time the
branches may break at the lesion. Pathogen associated with the affected samples were isolated
by using tissue segment method. Single spore isolation and hyphal tip techniques were
followed to get monoconidial isolates of pathogen. The pathogen associated with the disease
was identified as Pyricularia grisea.
Pathogenicity of isolates of Pyricularia grisea were confirmed under glasshouse
conditions and all the isolates were found pathogenic on susceptible rice cultivar TN1.
Significant differences were observed among the isolates of P. grisea in pathogenicity and
highest per cent disease index was observed in isolates Pg2 and Pg12 with PDI of 88.8 per cent.
The least PDI was observed in isolate Pg4 (48.1%). The rest of the isolates PDI was ranged
from 85.1 to 66.6 per cent. The individual isolates of pathogen were re-isolated from the
inoculated plants and proved the Koch’s postulates.
Significant differences were observed in colony characters viz., radial growth, colony
color, growth pattern, texture of colony, sectoring, zonation, wrinkle formation, dry mycelial
weight, time of sporulation and sporulation index among the twelve isolates of P. grisea on
OMA, PDA and host leaf extract + 2% sucrose agar medium after 14 days of incubation. The
highest mean radial mycelial growth of the fungus was recorded on oat meal agar (81.7 mm)
followed by Potato dextrose agar (77.8 mm) and least mean radial mycelial growth of the P.
grisea isolates were recorded on host leaf extract + 2% sucrose agar (72.5 mm).
Colony colour of twelve isolates of P. grisea were differed from greyish white to
greyish black on three solid media tested. All the isolates were circular form and varied with
respect to mycelium elevation and texture. Significant differences were also observed among
the isolates with the formation of sector, zonation and wrinkles.
Among the three different liquid media tested, highest mean mycelial dry weight of the
P. grisea isolates was recorded on potato dextrose broth (225 mg) followed by oat meal broth
(214 mg) and least mean mycelial dry weight on host leaf extract + 2% sucrose broth (164 mg).
Time taken for sporulation of P. grisea isolates on OMA medium was 7.9 days followed by
host leaf extract + 2% sucrose agar medium 8 days and PDA medium 8.2 days. Sporulation
index of twelve P. grisea isolates were varied from poor to excellent on rating scale of 1 to 4
on three solid media tested.
Conidia of the isolates were produced in clusters on long septate, slender
conidiophores. Significant differences were observed with conidial size of P. grisea isolates
on OMA medium. The mean conidial size ranged from 18.9 to 28.2 μm in length and 6.1 to 9.3
μm in width among twelve P. grisea isolates. The shape of conidia in all the isolates was
pyriform and hyaline to pale olive, 2 septate and 3 celled. Spore germination percentage was
high in Pg1 isolate (91.6 %) and least in Pg6 isolate (28.3 %).
Studies on pathogenic variability of P. grisea isolates on 25 international host
differentials revealed that significant variations were observed in respect of leaf blast severity,
virulence pattern and per cent disease index (PDI) among the isolates. The results indicated
that Pg5 isolate from Nizamabad district was found more virulent with mean leaf blast severity
of score 4.2 by producing symptoms in 19 out of 25 host differentials and exhibiting per cent
disease index (PDI) of 47.1 per cent. Whereas Pg1 isolate from Karimnagar district was least
virulent with minimum leaf blast severity of 2.8 by showing susceptible reaction on 8 host
differentials out of 25 host differentials and exhibiting per cent disease index of 28.2 per cent.
Cluster analysis of the isolates revealed that all P. grisea isolates were divided into six
pathotype groups.
The relativity of isolates of P. grisea that represent the wide collection of races from
Telangana State were examined. A total of 5 races were detected among 12 isolates. The most
frequently occurred race was IA (Pg2, Pg5, Pg7, Pg10 and Pg12) followed by IB (Pg6 and
Pg11), IC (Pg3 and Pg8), ID (Pg4 and Pg9), IG (Pg1).
Molecular variability among the isolates of P. grisea from different locations were
detected by using a total of ten SSR markers. Of these, eight (80 %) SSR markers showed
polymorphism among P. grisea isolates and identified a total of 19 alleles among the isolates.
The number of alleles amplified by each primer pair ranged from 2 to 3 with an average of 2.37
alleles per locus. The PIC (Polymorphic Information Content) value for P. grisea populations
were ranged from 0.30 and 0.81.
The cultural, morphological, pathogenic and molecular studies revealed that out of
twelve P. grisea isolates only three isolates viz., Pg3 (Mancherial), Pg7 (Peddapalli) and Pg9
(Khammam) districts showed correlation with respect to radial mycelial growth, mycelial dry
weight and time taken for sporulation, whereas correlation was not found among other nine P.
grisea isolates. Results also revealed that there was no association between grouping of the P.
grisea isolates based on pathogenic and molecular data as molecular polymorphism is largely
independent of virulence polymorphism. This is expected because genes controlling a
particular character is most likely to be present in a small fraction across the genome, whereas
the molecular banding pattern obtained from the total DNA reflects diversity in the entire
genome.
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D10,492