SUMMARY AND CONCLUSIONS
Rhizoctonia solani has long been recognized as a destructive pathogen on a wide variety of
plants throughout the world. The disease caused by Rhizoctonia solani (Kuhn.), sheath blight of rice is
considered as major bottleneck in successful production of rice in India, especially in irrigated tract of
Tungabhadra Project area of North Eastern Zone of Karnataka. Results of the investigations
undertaken on variability of the pathogen and management is summarised here under.
The study comprises of survey on prevalence and incidence of sheath blight of rice, collection
of sheath blight pathogenic isolates for exploring cultural, morphological and molecular variability,
evaluation of rice genotypes, influence of planting density, nitrogen level, botanicals and biocontrol
agents and IDM against sheath blight of rice.
The maximum incidence of sheath blight was recorded in Bellary district (33.83%) followed by
Koppal (32.90%) and Raichur (31.40%), while the minimum incidence was recorded in Mysore
(21.90%) followed by Shimoga (22.14%) and Mandya (26.62%) during kharif 2006 and 2007.
Additional information on sheath blight incidence with respect to dosage of nitrogenous fertilizer
applied was obtained and averaged for the fields receiving same dosage. Expectedly, the per cent
sheath blight incidence was maximum in some fields of Gangavati (37.79%) of Koppal district, which
received very high nitrogenous fertilizer dose (175-250 kg/ha) with an average of 226.25 kg/ha. The
low incidence of sheath blight was noticed in some fields of Manvi (23.74%) of Raichur district, which
had received moderately high level of fertilizer (120-175 kg N/ha) dosage of N with an average 121.5
kg per ha.
The symptoms appeared as spots which are greenish grey, ellipsoid ovoid and about one cm
long. They enlarged and reached 2 or 3 cm in length and became greyish white with brown margins
and some what irregular in outline. The fungal mycelium spread to other leaf sheaths and blades.
Eventually, the whole sheath rots and the affected leaf can easily be pulled off from the plant. In
severe cases, all the leaves of plant are blighted resulting in death of the plant. The rice plants show
such blighting in groups and patches.
Thirty two isolates of R. solani were obtained from Raichur, Bellary, Koppal, Mandya, Mysore
and Shimoga in Karnataka and Hyderabad in Andhra Pradesh, Kapurthala and Amritsar of Punjab,
Hissar of Haryana and Coimbatore of Tamil Nadu.
The fungus on PDA produced rapidly grown mycelium pale to dark brown with relatively large
diameter with branching near the distal septum of the hyphal cell, constriction of the branch hyphae
was found at the point of origin with formation of the septum. Mycelial branching was right angled and
barrel shaped cells called monilioid were produced. Sclerotia were superficial more or less globose
but flattened below, white when young became brown or dark brown later. Further, the isolates were
identified as R. solani based on morphological features and their identity was confirmed by Principal
Mycologist, National Academy for Fungal Identification, New Delhi. Further, isolates were subjected to
Koch's postulates and identified as R. solani.
In cultural studies, local isolate RS-20 was supported by PDA with maximum radial growth
followed by oat meal and host extract agar. On the contrary, PDA supplemented with CaCO3 failed to
generate any radial growth of the fungus. Least radial growth of the fungus was observed on
Sabourd's media. Further, fungus recorded moderate to low number of sclerotia on various media.
Among different media, PDA supplemented with CaCO3 media failed to initiate any sclerotial
formation.
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The maximum dry mycelial weight was observed on potato dextrose broth on 13 day after
0
incubation and thereafter started declining in dry mycelial weight. A temperature of 25 C was found to
be favourable for good germination of sclerotia. The least number of sclerotial germination was
observed at a temperature of 15 °C.
The diversity with respect to morphological and cultural characters of 32 isolates of R. solani
was studied on PDA. There existed much variation among the isolates, in mycelium colour, growth
pattern, type of margin, sclerotial number, colour, shape and texture, growth pattern and formation of
sclerotia etc. The branching of mycelium, constriction and septum remained same in all the isolates
tested. Among 32 isolates, maximum radial growth of mycelia was noticed in isolates RS-14, RS-16
and RS-18 each with 89.33 mm. The least mycelial growth was recorded in isolate RS-3 and RS-26
with 79.33 mm. The number and size of sclerotia also varied among the isolates. The isolates RS-3,
RS-4, RS-7, RS-9, RS-10, RS-15, RS-19, RS-21, RS-27, RS-30, RS-32 recorded higher number of
sclerotia. Whereas less number of sclerotia was observed in isolates RS-2, RS-12, RS-14, RS-17,
RS-20, RS-22, RS-23 and RS-25 except RS-1, RS-6, RS-24 and RS-29 which failed to produce any
sclerotia on the PDA medium. Further, among the isolates mycelial width was larger in the isolate RS13 and RS-24 (2.72-2.84 µm), whereas, least mycelial width was observed in RS-2 (1.65-2.01 µm).
The maximum sclerotial size was observed in isolate RS-17 and RS-28 (180 µm), whereas, least
sclerotial size was seen in isolate RS-7 (46.6 µm). Based on the morphological and cultural
characteristics, the isolates of R. solani were grouped into four and designated as G1, G2, G3 and
G4. There were G1 group comprised of RS-1, RS-2, RS-5, RS-6, RS-8, RS-12, RS-16, RS-18 and
RS-22, G2 isolates (RS-3, RS-4, RS-7, RS-9, RS-10, RS-15, RS-19, RS-21, RS-27, RS-28, RS-31
and RS-32), G3 isolates (RS-14, RS-17, RS-20, RS-23, RS-24, RS-26, RS-29 and RS-30) and G4
isolates (RS-l1, RS-13 and RS-25).
The molecular analysis of variability using RAPD primer indicated that, all isolates shared 60
per cent and above similarity among themselves. The cumulative analysis of 32 isolates showed 4
groups at 75 per cent genetic similarity level. As per dendrogram cluster analysis, all the 32 isolates
were grouped into two major clusters such as C1 and C2, wherein cluster C1 consists of 15 isolates
and cluster C2 consists of 17 isolates. The main cluster C1 has been further grouped into two subclusters C1-1 and C1-2. Cluster C1-1 consists of seven isolates namely RS-1, RS-5, RS-9, RS-15,
RS-24, RS-16, RS-22 and the cub-cluster C1-2 consists of 8 isolates namely RS-25, RS-21, RS-31,
RS-30, RS-14, RS-29, RS-20 and RS-13. The main cluster C2 consists of 17 isolates. The sub-cluster
under main cluster C2-1 consists of 10 isolates namely RS-2, RS-3, RS-4, RS-32, RS-18, RS-17, RS28, RS-23, RS-11 and RS-10, whereas the sub-cluster C2-2 consists of seven isolates namely RS-6,
RS-26, RS-7, RS-8, RS-27, RS-19 and RS-12
Among 139 rice genotypes screened against R. solani in field under natural epiphytotic
condition at ARS Siruguppa, none of them were found immune against R. solani. However, five
genotypes viz., Aditya, Ajaya, Swarnadhan, Nidhi and Vikramarya were found resistant by scoring 1
grade. Whereas, three genotypes viz., Swarna, IR-64, Mandya Vijaya were found to be moderately
resistant by scoring 3 grade, Remaining 72 were moderately susceptible scoring 5 grade. Remaining
52 were susceptible and seven were found highly susceptible by scoring 7 and 9 grades, respectively.
Genotypes which showed resistant and moderately resistant ones under field condition were
artificially inoculated by R. solani using "straw bits method" in pot culture experiment under growth
chamber and "sheath inoculation technique". Aditya, Nidhi, Ajaya, Swarnadhan and Vikramarya
showed resistance with grade 1 whereas Swarna showed moderately resistance with grade 3, IR-64
and Mandya Vijaya showed moderately susceptible each with grade 5, respectively. Even in sheath
inoculation technique also the reaction of genotypes was similar to that of straw bits method.
In vitro evaluation of ten systemic fungicides by ‘Poisoned Food Technique’ against R. solani
revealed that maximum per cent inhibition of mycelial growth (100%) was observed in hexaconazole
(Contaf 5 EC), propiconazole (Tilt 25 EC), carbendazim (Bavistin 50 WP), benomyl (Benlate 50 WP)
and thiophanate-methyl (Topsin-M 70 WP) followed by Kitazin 48 EC (91.26%). Whereas,
difenconazole 25 EC at 0.05 per cent was found to be least effective (70.56%) in inhibition of mycelial
growth of R. solani.
Among botanicals, Achook (Azadirachtin 0.15%) with 73.79% was very effective with 73.79
per cent mycelial growth inhibition which was followed by Tricure (Azadirachtin 0.03%) with 66.31%
and NSKE (@ 5%) with 47.71%. The least inhibition was recorded by Nimbicidin (Azadirachtin 0.15%)
with 21.93% followed by neem oil (26.22%).
Evaluation of bio-agents under laboratory conditions by using dual culture method against R.
solani revealed that among the fungal bio-agents Trichoderma viride (Tv-16) isolate was found to be
effective with 75.50 per cent inhibition followed by Trichoderma viride (Tv -41) with 74.78% and
Trichoderma viride (Dharwad) recording 70.70%. While, Trichoderma harzianum (Th-10) isolate was
found to be least inhibiting of R. solani (58.80%). Among bacterial bio-agents, P. fluorescens (Pfr-1)
was found to be effective inhibiting R. solani. The lowest inhibition (10.67%) was noticed in Bacillus
subtilis (Dharwad). Whereas, both Coimbatore and PGPR isolates of Bacillus subtilis completely failed
to inhibit the mycelial growth of R. solani.
The best fungicides, botanicals and bioagents identified in vitro were imposed under field
condition on Cv. Samba mahsuri. Among the different treatments, the least disease incidence (17.00
PDI) was observed in Hexaconazole (Contaf 5 EC) with the highest yield recorded (81.02 q/ha). This
was followed by Validamycin 3L (21.60 PDI and grain yield 73.83 q/ha) and Carbendazim 50 WP
(24.80 PDI and grain yield 69.21 q/ha). Among the botanicals and bio-agents, the least disease
incidence was noticed in Tricure (Azadirachtin 0.03%) with 30.50 PDI and grain yield of 61.43 q per
ha and P. fluorescens (Pfr-l) with 36.20 PDI with a grain yield of 54.02 q per ha.
The yield and subsequent net income was more in Hexaconazole (Contaf 5EC) (Rs. 31,825)
which gave the cost to benefit ratio of 1:2.5 and was found to be best, followed by Validamycin 3L
with a cost benefit ratio of 1:2.3.
Out of 139 genotypes screened against sheath blight of rice, none of them were found to be
immune. However, five genotypes viz., Nidhi, Ajaya, Vikramarya, Swarnadhan and Aditya showed
resistant reaction by scoring grade 1.
Among different nitrogen levels, application of 200 kg N per ha recorded higher PDI of 34.7
per cent. However, increased N application of 350 kg N per ha resulted in increased PDI (58.44%).
Minimum PDI was recorded at 150 kg N per ha. The incidence was negligible (2.9 PDI) when no
nitrogenous fertilizer was applied.
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Among different planting densities, the SRI method (16 hills/m ) recorded least PDI (3.70%)
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whereas increased planting density from 20 to 70 hills per m resulted in higher PDI from 7.04 to
47.03 per cent, respectively.
In IDM trial, use of cultivar IR-64 with application of hexaconazole resulted in minimum PDI of
21.26% and higher grain yield of 71.74 q per ha as against 34.73 PDI and grain yield of 56.87 q/ha in
cultivar Samba mahsuri.
Future line of work
1.
Work on induced systemic resistance using both chemicals and bioagents to activate the
defense genes for furthering eco-friendly management of sheath blight.
2.
Use of PR-proteins for development of resistance against sheath blight of rice using
transformation.
3.
Screening and identification of novel resistance sources.
4.
Anatomosis grouping (AG) and their virulence profile.