Sheath Blight (ShB) (C.O. Rhizoctonia solani Kühn) is one of the most devastating diseases of rice particularly in wet season. The disease has spread in all rice growing areas in large scale and in some areas it is due to widespread cultivation of susceptible variety, as in case of West Bengal and Odisha where ‘Swarna’ (MTU 7029) which is widely cultivated and is highly susceptible to sheath blight pathogen ‘R. solani’. Damage is estimated upto 100% in favourable climatic condition and yield loss of rice varies from 5.2-50% depending on disease severity [1-3]. Though cultivation of resistant variety is the best option to cope up the attack of this pathogen but till date no such variety is available to the growers. Thus, in present situation cultural practices combined with foliar spray of fungicide is the most common practice to manage the disease and even in integrated pest management system need based application of fungicide has been recommended. In this context, a long term experiment was conducted to evaluate the efficacy of new generation bio-rational fungicide of ‘strobilurin derivatives’. Strobilurins (also known as β-methoxyacrylates) or QoI (Quinone outside Inhibitors) fungicides, launched in 1996 are analogues of strobilurin-A [4] originally derived from natural products [5] (Strobilurus tenacellus, a wild mushroom growing in forests). Strobilurins, are now the second largest chemistry group of fungicides, widely used on cereals and, more recently, on soybeans (a market that reached $600 million in 2004). The strobilurin fungicides mainly act on mitochondrial synthesis in cytochrome bc1, are highly effective, and are suitable for a wide range of crops include Azoxystrobin, Fluoxastrobin, Kresoxim-methyl, Metominostrobin, Picoxystrobin, Pyraclostrobin and Trifloxystrobin (Table 1). The new fungicides were most effective in decreasing disease severity and increasing grain yield in comparison to control (without fungicide). These fungicides were also proved as best or at par with leading triazole compounds to manage the sheath blight disease of rice at several collaborating and volunteer centre under All India Coordinated Rice Improvement Programme (AICRIP) [6,7].

Table 1: Different strobilurin analogues and their use.

Systematic research was initiated since 2002 and followed up to 2010 at experimental farm of Rice Research Station, Chinsurah, West Bengal (22°89′N and 88°39′E) to evaluate various strobilurin and combination of strobilurin fungicide viz. azoxystrobin 25 SC, trifloxystrobin 25%+tebuconazole 50% 75 WG and Metaminostrobin 20 SC. In each experiment, new fungicide, 2-3 commercial fungicides, control (without fungicide) and one best commercial fungicide (check fungicide) were taken as treatments. All the experiments were conducted with rice variety ‘Swarna’ (MTU 7029) following standard agronomic practices except using higher nitrogenous (120 kgha^-1) and lower pottasic (30 kgha^-1) fertilizer dose than the normal dose (N₂:P₂O₅:K₂O::100:50:40). For confirming appearance of disease, artificial inoculation with virulent strain of ShB pathogen R. solani were performed following ‘Straw-bit’ method [8] during active tillering stage. Fungicides were sprayed thrice at an interval of 10 days starting from the initial appearance of the disease. The final disease incidence was recorded 15 days after last spray from 20 randomly affected hills in each treatment and the plants were assessed individually using SES scale (0-9 scale) [9].

It is evident from all the experiment that new strobilurin fungicides are effective in checking sheath blight disease and further spread of the pathogen when used as a sole chemical or in combination with other triazole compound (Table 2).

Table 2: Bioefficacy of new fungicide against sheath blight disease.

It was observed that azoxystrobin 25 SC @ 1 ml l^-1 effectively controlled the disease (having the lowest disease severity 16.4%) and improved grain yield (max. 5225 kgha^-1) [10]. ‘trifloxystrobin 25%+tebuconazole 50% 75 WG’ @ 0.4g l^-1 -a combination of two systemic fungicides like strobilurin compound ‘trifloxystrobin’ and triazole compound ‘tebuconazole’ along with two effective commercial fungicides viz., Hexaconazole 5 EC and Validamycin 3L were taken as commercial check fungicides for the experiment. Disease severity was 37.61% lower and yield 50% higher over untreated (control) [11]. Another experiment revealed that Metaminostrobin 20 SC @ 2 ml l^-1 was superior to any other popular check fungicides. This product reduced the disease severity by 64.7% lower and yield 40.5% higher over untreated (control) [12]. Various experimental reports confirmed that strobilurin compounds found to be effective in controlling other rice diseases like blast [13], grain discoloration [14], sheath rot and brown spot etc. [15]. In a recent report it was evident that not only fungal diseases, viral disease like TMV infection is also restricted in Tobacco plant pre-treated with pyraclostrobin [16]. Though several fungicides like carbendazim, validamycin and several triazoles like propiconazole, hexaconazole have been recommended for sheath blight disease management but in areas which regularly suffer with severe sheath blight disease, application of any strobilurin fungicides viz. azoxystrobin 25 SC @ 1 ml l^-1, trifloxystrobin 25%+tebuconazole 50% 75 WG’ @ 0.4g l^-1 or Metaminostrobin 20 SC @ 2 ml l^-1 may be advocated for recommendation for management and may include in IPM for improving of the grain yield which has ultimately enhance the productivity of rice in West Bengal [17]. Strobilurin compound based fungicides are formulated using very less active ingredient and are eco-freindly, possessing desired fungi-toxicity, good light stability, systemic properties without phytotoxicity and safe to use in plant disease control.