Standardization of inoculation techniques for sheath blight of rice caused by Rhizoctonia solani (Kuhn)

To develop a simple and reliable inoculation technique using inoculum sources viz., mycelial suspension, mycelial ball, sclerotia and soil inoculation with homogenized mycelia suspension was carried out. The efficiency of different inoculation techniques were tested on susceptible rice variety Rejendra Sweta. Sheath inoculation with sclerotia gave lesion length 10.33 and 12.33 cm after seven and 15 days of inoculation, respectively. It also shows more significance in terms of relative lesion height to plant height i.e. 30.06 % followed by soil inoculation with homogenized mycelial suspension i.e. 21.62 %. However, relative number of lesions as compared to control was found to be maximum (429.18%) in mycelial suspension spray, followed by soil inoculation using homogenized mycelia suspension (400.43%). The disease rating of sheath blight of rice was also found to be maximum (5) in sheath inoculation with sclerotia followed by soil inoculation of mycelial suspension (3). The disease rating was minimum (1) in sheath inoculation with mycelial ball and foliar spray with mycelial suspension. Introduction Rice is grown as an important cereal crop all over the world, but extensively grown in southeast Asian countries. Sheath blight of rice, a disease caused by Rhizoctonia solani Kuhn (teleomorph: Thanatephorus cucumeris (A.B. Frank) Donk) has become a most devastating plant pathogen, which causes yield losses up to 54.3% (Chahal et al. 2003) in rice growing tracts of India. It is considered as one of the most important factors for stagnated productivity of the crop in the country. R. solani is soil-borne plant pathogen and has a wide host range, often infecting legume crops grown in rotation with rice (Rush and Lee 1992, Zou et al. 2000). Initially the symptoms appear as water soaked chlorotic patches on leaf sheaths at culms just above the water level which are ellipsoid or ovoid about 10 mm long somewhat irregular and greenish grey, they enlarge in size and reach 1 3 cm long with irregular margin. Many different inoculum sources including colonized agar plugs, colonized rice grain or rice straw mixtures blended mycelia, sclerotia has been used in the past for investigating rice sheath blight (Gangopadhyay and Chakraborty 1982, Willocquet et al. 2000, Eizenga 2002). Most of these inoculum sources introduce considerable variability in terms of infection process. Each of these inoculum types has been used by applying different inoculation methods, either in the field or under controlled conditions. In addition, a diversity of inoculum sources and methods used for the analysis of sheath blight resistance, different techniques have been applied for evaluating the severity of disease. The present study has been, undertaken to develop an effective, uniform, and reproducible technique for infecting rice plants with R. solani and to improve the accuracy of disease severity evaluation system. This information may be used to devise a simple, less time consuming, precise and reproducible seedling inoculation technique for sheath blight of rice. *Author for correspondence: . KVK, Agwanpur, Saharsa, (Bihar Agricultural University, Sabour) 852 201, Bihar, India. Department of Molecular Biology and Genetic Engineering (Biotechnology), Bihar Agricultural University, Sabour (Bhagalpur)-813210, Bihar, India.