Effect of fungicides and non-conventional chemicals on Rhizoctonia solani AG-1 1A and sheath blight disease of rice

In-vitro growth inhibition test revealed that the fungicides varied considerably in their toxicity to Rhizoctonia solani. The EC₅₀ values of fungicides ranged from 0.18 to 39.81 ppm a.i., whereas EC₉₀ values were between 1.66 to >500 ppm a.i. Hexaconazole, carbendazin and trifloxystrobin + tebuconazole proved most toxic while metominostrobin and azoxystrobin were least toxic to fungal growth of R. solani. These fungicides also varied considerably in their ability to inhibit formation of sclerotia. The complete inhibition of sclerotial formation was observed at 2.0 ppm a.i. of hexaconazole, 2.5 ppm a.i. of carbendazim and >10 ppm a.i. of trifloxystrobin + tebuconazole and tebuconazole. Azoxystrobin and metominostrobin failed to provide complete inhibition of sclerotial formation even at concentration of 500 ppm a.i. In a field trial, all the fungicides reduced disease severity and increased grain yield of cultivar HKR 127 significantly. Azoxystrobin proved best with >70 per cent reduction in relative lesion height (RLH) closely followed by trifloxystrobin + tebuconazole. Hexaconazole and tebuconazole reduced RLH by >50 per cent. All the non-conventional chemicals tested in vitro, reduced growth and sclerotial formation of R. solani significantly at a concentration of 5 mM and more. The lowest concentration (2 mM) of lithium sulphate, nicotinic acid, 4-aminobutyric acid and thiourea had non-significant effect on growth and sclerotial formation while the highest concentration (20 mM) of propionic acid, o-chlorobenzoic acid and isobutyric acid resulted in complete inhibition of growth and sclerotial formation. Nicotinic acid and 4-aminobutyric acid had least effect on growth and sclerotial formation of the fungus at highest concentration tested. In field, all the non-conventional chemicals reduced the relative lesion height to some extent (13.4–24.6%) during both the years of testing; however the increase in yield was significant only in plots sprayed with isobutyric acid, lithium sulphate, 4-aminobenzoic acid and propionic acid.