~ 691 ~ ISSN Print: 2617-4693 ISSN Online: 2617-4707 IJABR 2024; 8(4): 691-701 www.biochemjournal.com Received: 02-01-2024 Accepted: 06-02-2024 Ashwini KS Department of Plant Pathology, College of Agriculture (UASB), V.C. Farm, Mandya, Karnataka, India Kiran Kumar N Department of Plant Pathology, College of Agriculture (UASB), V.C. Farm, Mandya, Karnataka, India Vijaykumar L Department of Entomology, College of Agriculture (UASB), V.C. Farm, Mandya, Karnataka, India Yogananda SB Department of Agronomy, College of Agriculture (UASB), V.C. Farm, Mandya, Karnataka, India Sanath Kumar VB Department of Plant Pathology, College of Agriculture (UASB), V.C. Farm, Mandya, Karnataka, India Corresponding Author: Ashwini KS Department of Plant Pathology, College of Agriculture (UASB), V.C. Farm, Mandya, Karnataka, India Defense reaction and biochemical interpretation of rice genotypes to sheath blight, Rhizoctonia solani Kuhn Ashwini KS, Kiran Kumar N, Vijaykumar L, Yogananda SB and Sanath Kumar VB DOI: https://doi.org/10.33545/26174693.2024.v8.i4i.1030 Abstract Rice (Oryza sativa L.) is one of the most important cereal crops in the world. Sheath blight disease of rice caused by Rhizoctonia solani Kuhn AG-1 IA (Telomorph, Thanatephorus cucumeris Frank. Donk) is a devastating disease in all rice-growing regions of the world. The yield losses ranging from 4-50% have been reported depending on the crop stage at the time of infection, the severity of the disease, and environmental conditions. Landraces are valuable genetic resources to explore novel genetic variations and they are highly adaptive. Considering the significance of this disease the present study was conducted at College of Agriculture, V.C. Farm, Mandya during Kharif 2021 and Summer 2022. Ninety landraces and ten popular varieties of rice were screened against sheath blight disease by artificial inoculation method. Disease scoring was done by the standard evaluation system (SES) as per IRRI (2002). The mean percent disease index (PDI) ranged from 11.11 to 79.56%. Based on AUDPC values, rice genotypes were categorized into 5 groups i.e., resistant, moderately resistant, moderately susceptible, susceptible, and highly susceptible. Further to know the resistance mechanism, 34 selected genotypes were analyzed for biochemical constituents by using standard procedure and protocols and it was found that phenol (-0.79), total soluble sugar (-0.84), reducing sugar (-0.87), crude protein (-0.84), tannin (-0.91), peroxidase (-0.67), polyphenol oxidase (-0.83), and phenylalanine ammonia lyase (- 0.84) was negatively correlated with sheath blight disease severity and these biochemicals were higher in resistance genotypes viz., Kalanamak and Sidda sanna compare to susceptible genotypes. Keywords: Sheath blight, rice, artificial inoculation, screening, resistance mechanism, Rhizoctonia solani Introduction Rice (Oryza sativa L.) is a member of the Poaceae family and belongs to the genus Oryza. Rice is grown in about 113 countries around the world. It is the primary food crop of the countries of the south and southeast. As the theme "Rice is Life" suggests, rice is the single most important staple food crop for more than one-third of the world's population and more than half of India's population. It is the staple food for more than two-thirds of the Indian population, and as such, it holds the key to food security and plays an important role in the national economy. Rice demand is expected to rise indefinitely as the world's population grows (Mahajan et al. 2017) [29] . In India, rice is grown under a variety of agro-ecological conditions (Maclean 2002) [28] . Insect pests, bacterial, viral, and fungal pathogens, as well as abiotic stresses such as ozone, heat, UV-B, heavy metals, and others, all pose a significant threat to rice yield. Rice sheath blight (ShB) is a fungal disease that has become a major concern in recent decades (Molla et al. 2020) [35] . The rice sheath-blight (ShB) pathogen, Rhizoctonia solani (Kuhn) Thanatephorus cucumeris (Frank.) (Donk), is a major production constraint for rice in India and most rice-growing Asian countries (Devi et al. 1989) [12] . Sheath blight of paddy is one of the most common paddy diseases. Miyaki was the first to report this disease from Japan in 1910. However, this disease was first reported in India by Paracer and Chahal in Gurdaspur (Punjab) in 1963. The disease's severity has increased due to intensive and changed cultivar practices. It is a potentially devastating rice disease in all temperate and tropical rice production regions, particularly in irrigated systems International Journal of Advanced Biochemistry Research 2024; 8(4): 691-701