Impact of fungicides on Rhizoctonia solani Kuhn causing sheath blight disease of rice

Rhizoctonia solani Kuhn, is an important pathogen of rice causing sheath blight disease. The pathogen thrives in soil and is polyphagous in nature. Till date no resistant variety has been registered against the disease which is the main reason of dependence on chemical management of the disease. In the present study six chemicals with different concentrations viz. 50 ppm, 100 ppm and 200 ppm along with control were evaluated in vitro against the pathogen. The chemicals were also tested in vivo for management of sheath blight. The in vitro study was conducted using the ‘poisoned food technique’ and the in vivo study was done in the field condition during kharif season in two subsequent years (2018 and 2019). Spraying of the fungicides was advocated twice after the natural infection i.e. at 45 and 60 days after transplanting. The observations revealed that all the fungicides were capable for significant inhibition of the fungus in ‘in vitro’ and the disease in ‘in vivo’ conditions.


Introduction
Sheath blight of rice is considered as the second most economically important disease of rice after blast (Manibhusanrao, 1995) [3] . It is mainly aggravated in the field when the crop is in active tillering stage. In Odisha, the crop is mostly affected by the disease during late September to October as the weather is more favourable for spread of the disease. Sheath blight disease is favoured by the high humidity condition, water logging situation near the sheath, close spacing and indiscriminate use of nitrogenous fertilizer (Kagale, 2004) [7] . Monocropping with high yielding varieties for several years is also one of the causes of prevalence of the disease due to genetic homogeneity. No resistant variety till now is registered against the disease. By observing the degree of incidence of the disease in favourable weather conditions, chemical management as the last tool under Integrated Disease Management (IDM) is very essential sometimes as the rate of spread of the disease is very high crossing Economic Threshold Level within a short span of time. Keeping in view the disease severity, a study was undertaken with some commercially available new generation chemicals along with standard chemicals, which were previously reported efficient against the disease, both in vitro and in vivo to evaluate their efficacy against the disease.

Materials and Method
In vitro Study of Chemicals against Rhizoctonia solani An in vitro study was conducted by taking six chemicals along with control (untreated) viz. (Tebuconazole 50% + Trifloxystrobin 25%) 75 WG, Propiconazole 25% EC, Azoxystrobin 25% SC, Validamycin 3L, Carbendazim 50% WP, Hexaconazole 5% SC in different concentrations of 50 ppm, 100 ppm and 200 ppm using the 'poisoned food technique' (Nene & Thapliyal, 1982) [6] . The media used for the purpose was potato dextrose agar. After calculation, the desired amount of chemicals were weighed and mixed with the calculated amount of the molten media to get the desired concentration. First 1% tween 20 was added to the emulsified chemicals for their thorough mixing and then poured into the molten media. The poisoned media with different concentrations were poured in 90 mm petri dishes. 5 mm mycelial discs were cut from the edges of 5 days old fresh culture plates of Rhizoctonia solani and inoculated at the centre of the plate.
Each treatment was replicated thrice and the design of the experiment was Completely Randomized Design (CRD). The whole process was carried out in aseptic condition by taking utmost care. The plates were incubated at 27 ± 1 °C for 24 hours and observations were taken at regular interval. The untreated plate with the fungus was maintained along with treated plates as control. The final observations were taken when the control plate attained its maximum mycelial growth and the treated plates were compared with the control plate to evaluate the inhibition of radial growth by the following formula (Vincent, 1927) [17] . Azoxystrobin 25% SC 1ml Triazole

Bio-Efficacy Study of Chemicals against Rhizoctonia solani under Field Condition
After observing efficacy of the chemicals against the pathogen in vitro, a field experiment was conducted in the Regional Research and Technology Transfer Station (OUAT), Ranital, Bhadrak for two consecutive years i.e. 2018 kharif and 2019 kharif to study the efficacy of the chemicals in field condition. The design of the experiment was Randomized Block Design (RBD) with seven treatments along with the control having three replications for each treatment. The rice variety 'Swarna (MTU 7029)' was taken as it is susceptible to sheath blight disease. The plot size was (6X3) m 2 . Standard agronomic practices with judicious fertilizer application were followed in the experiment. The crop was allowed to have natural incidence of the sheath blight infection. The chemicals were sprayed twice in respective plots at 15 days interval at active tillering stage (45 DAP) and at 60 DAP. The control plot was sprayed with only water. The data on disease incidence were recorded

Statistical Analysis
The observations recorded in the various experiments of the study were statistically analysed by OPSTAT agriculture data analysis tool, CCSHAU, Hissar.

Results and Discussion
The present in vitro study on the chemicals showed that all the chemicals significantly reduced the mycelial growth at all the concentrations. The inhibitory effect of all test chemicals increased with the increase of concentrations.  [14] reported the combination of strobilurin and azole compounds in (Tebuconazole 50% + Trifloxystrobin 25%) 75 WG could affect the quinines and sterol biosynthesis in the fungal pathogen. The effect of Hexaconazole 5% SC and Propiconazole 25% EC was also found significant with least PDI 28.99% and 26.81% with an increase in yield by 27.29% and 22.19% over control, respectively. Kumar et al. (2014) [10] and Chandra et al. (2016) [2] reported Propiconazole 25% EC at 1ml/l as most efficient with disease severity of 7.77% and 7.50% with gain yield of 3486.04 kg/ha, 3985 kg/ha, respectively. Likewise Swamy et al. (2009), Bhuvaneswari and Raju (2012) reported Hexaconazole at 2ml/l to be limiting the disease severity within 25% and 31.06% with gain yield of 5087 kg/ha and 6661 kg/ha, respectively. The promising effect of Hexaconazole 5% SC and Propconazole 25% EC also corroborates with the finding of Nagaraju (2013). This was followed by Carbendazim 50% WP with PDI 31.84% and Validamycin 3L with PDI 32.24% with grain yield of 47.77 q/ha and 44.97 q/ha, respectively and were found to be at par with each other. The finding was supported with the work of Nagaraju et al. (2017) [5] who found the effect of Carbendazim 50% WP and Validamycin 3%L to be at par in their performance with PDI of 24.80% & 21.60% with higher grain yield of 69.21% and 73.83%, respectively in field trial but contrast to the finding of Prakash (2015) i.e. Carbendazim 50% WP is most effective against the disease with low PDI of 24.81%. The effect of Azoxystrobin 25% SC on the fungus was not that promising (with PDI 24.98 and grain yield 45.75 q/ha) in the finding of Bag et al. (2016). The report revealed that Azoxystrobin 25% SC @1 ml/l effectively controlled the disease (with the lowest disease severity 16.4%) and improved grain yield 5225 kg/ha.