STUDIES ON VARIABILITY OF SHEATH BLIGHT OF RICE CAUSED BY Rhizoctonia solani Kuhn. AND ITS MANAGEMENT
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Date
2015
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PROFESSOR JAYASHANKAR TELANGANA STATE AGRICULTURAL UNIVERSITY. HYDERABAD
Abstract
Rice (Oryza sativa L.) is one of the three major food crops of the world and is
the staple food for more than half of the world’s population. It is an important cereal
crop affected by various fungal, bacterial and viral diseases Sheath blight of rice caused
by Rhizoctonia solani (telomorph: Thanatephorus cucumeris) has become a major
constraint to rice production during the last two decades.
Twenty R. solani isolates were collected from sheath blight diseased samples
collected from major rice growing areas of Karnataka state, India were designated as
RS-K-1 to RS-K-20. Pathogenicity of all the isolates was proved by Koch postulates by
artificial inoculation on susceptible cultivar TN-1.
Studies morphological variability revealed that all the isolates of R. solani
characteristically branched at right angle in the distal end of the cell. The hyphal width
of all the twenty isolates varied from 1.80 to9.43 μm. The minimum distance between
two septation was 15.37 μm, while maximum distance was 351.92 μm.
The cultural variability among twenty isolates showed that the colony color
varied from Ivory to pale brown. Among the 20 isolates, five isolates produced colonies
were Ivory, two were olive, nine were sand yellow and four were pale brown in color.
Besides the reverse color of the culture, isolates were classified into three categories
namely, Ivory, sand yellow and olive green. While the colony texture varied from
fluffy, flat plain and slightly fluffy mycelial growth. Based on the growth rate R. solani
isolates were categorized into slow growing (1.80-2.0 mm/h), moderately growing (2.0-
2.0 mm/h) and fast growing >2.2 mm/h.
Based on pattern of sclerotial production twenty R. solani isolates were grouped
into five categories viz., sclerotia grouped at centre(7), lower ring (3), middle ring (2),
peripheral ring (4) and scattered (4). Among 20 R. solani isolates nine isolates secreted
honey dew and others did not. However maximum number of sclerotia observed was
618 (RS-K-1), while minimum number of sclerotia was 67 (RS-K-5). The sclerotial
weight of twenty isolates ranged from 8.75 to 18.50 mg.
Pathological variability of twenty R. solani isolates was studied on susceptible
cv. TN-1 and found that isolates took 2-5 days to exhibit the typical sheath blight
symptoms. And the size of the lesion ranged from 0.15- 3.15 cm2, most of the isolates
produced either elliptical or elongated lesions. The disease severity varied from 17.26-
33.86 %, among the isolates. All the twenty isolates were classified highly virulent (
>50 % DI) based on per cent disease incidence.
Genetic diversity of R. solani isolates from different locations using 10 RAPD
and ISSR primers showed good polymorphism at DNA level and cluster analysis of
RAPD data grouped the isolates on the basis of their origin with few exceptions. A total
of 102 amplicons were obtained from twenty R. solani isolates with 10 RAPD primers
of which 100 % were polymorphic. Cluster analysis of sheath blight isolates revealed
the average pair-wise similarities in the range of 0.29-0.57 thus suggesting large
variations among the isolates. In case of ISSR primers a total of 111 were obtained with
10 ISSR primers of which 95.92 % were polymorphic.Cluster analysis revealed that 20
isoaltes grouped into two major clusters at 35 % genetic similarity coefficient.
Similarity coefficient ranged from 0.30-0.50 thus suggest good variations among the
isolates.
In vitro screening of eleven fungicides against R. solani revealed that
Trifloxystrobin + tebuconazole (0.4 g/l and 1.0 g/l), Captan + Hexaconazole (1.0 g/l),
Flusilazole + Carbendazim (0.5 ml/l and 1.0 ml/l), Hexaconazole (2.0 ml/l),
Azoxystrobin (1.0ml/l), Propiconazole (1.0 ml/l) Carbendazim (1.0g/l) and Validamycin
(2.5 ml/l) showed maximum inhibition. Out of eleven fungicides tested Azoxystrobin
was found to be the best in respect of reduction in disease incidence (34.4 %) and
enhancement of yield (5181 kg/ha). Azoxystrobin recorded low per cent disease index
(34.4 %) and maximum (100 %) in Metalaxyl-M + mancozeb (94.4 %) @ 1.0 g/l.
Among nine botanical extracts evaluated against R. solani Thevatia peruviana,
(86.61 %) exhibited strong fungitoxicity under in vitro conditions which recorded
lowest mycelial growth of 11.9 mm. the lowest PDI was recorded by Jatropha curcas
(34.73 %) with a yield of 4538 kg/ha. The highest yields were recorded in Chrozophora
tinctoria (4863 kg/ha).