First Report of Sclerotium hydrophilum Sace Causing Stem Rot Disease of Rice in North Eastern Plain Zone of India.

Abstract

High incidence of stem rot in rice (Oryza sativa L.) caused by Sclerotium hydrophilum was observed in Eastern Gangetic plains of India including eastern Uttar Pradesh and Bihar states in rice-wheat irrigated ecosystem in the months of August and September of the years 2016-17 at the maximum tillering stage of the crop plants. A survey was conducted for the prevalence of rice diseases in Eastern Uttar Pradesh and Bihar. Stem rot incidence (12-14%) was observed in widely cultivated rice varieties viz., Swarna, Sabour Surbhit, Samba Mahsuri and Bhagalpur katarani. Brownish to black water-soaked lesions without distinct margins were visually observed on rice leaf sheaths in Pusa, Bihar and Allahabad, Uttar Pradesh (India). To isolate the causal pathogen, pieces of symptomatic leaf sheaths were treated with 1% sodium hypochlorite solution for 1 min, rinsed with sterile distilled water for 2 min, and then transferred to potato dextrose agar (PDA) medium for incubation at 280C. The cultivated mycelium was transferred to fresh PDA medium. Colonies of pure cultures on PDA medium were initially white and turned brown about 2 weeks later. The hyphal width was measured with a range of 4.0 to 6.0 μm. Large numbers of small globose sclerotia were observed on surface of the colonies at 5 days after sub-culturing. The sclerotia were white at first and then turned black over time with maturity. The diameters of sclerotia ranged from 0.32 to 0.51 mm with an average of 0.41 mm (n = 50). DNA of a representative isolate named SH1 was extracted, and the ITS region was amplified by PCR with universal primer pair ITS1/ITS4 (White et al. 1990). Sequence analysis showed 99.66% identity with Sclerotium hydrophilum isolate VC228 (accession no. KT362098) and accession no. obtained (KX181457). Phylogenetic analysis based onneighbour-joining method grouped the isolatesalongwith other isolates from Asia. The pathogen was identified as Sclerotium hydrophilum on the basis of cultural, morphological, pathogenic and molecular characteristics. Further, culture was deposited to Indian Type Culture Collection (ITCC) at the Indian Agricultural Research Institute, New Delhi for the identification where it was identified as Sclerotium hydrophilum with identification number 10039.15. Another culture specimen of the pathogen was also deposited at the ITCC-IARI, New Delhi with accession no. 19160.25. Koch's postulates were completed by inoculating rice plants with PDA disks of5 mm in size bearing both mycelium and sclerotia of S. hydrophilum in 45 days old plants of rice genotype 'Samba Mahsuri' under glass house conditions. After 4th days leaf sheath necrosis was observed around the inoculation plugsunder field conditions.The same fungus was re isolated from the symptomatic lesions of inoculated plants, fulfilling Koch's postulates. Pathogen was reported with high disease incidence in Northern Karnataka (Pramesh et al. 2017). However, based on literature and fungus-host distribution records this is the first report of S. hydrophilum on leaf sheath of rice from North-Eastern plain zone of India.