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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.
In the summer season of 2016 and 2017, in some fields of Cauvery command area of Karnataka the rice crop showed stunted growth with brownish to black lesions without distinct margin on stem at the water level during tillering and reproductive phase . The causal organism was tissue isolated on potato dextrose agar, colonies of pure cultures showed white mycelium at first and after two days white coloured scelerotial formation started which later changed to chocolate brown and after five days became black colour. The hyphal width was measured to be within a range of 6 to 10 mm and binucleate hyphae were observed between the septa. Small, globose and reddish brown to black coloured mature sclerotia measuring 0·24-0·43 mm were observed in culture. Koch's postulates was proved by inoculating five days old culture disc containing sclerotia on the stem of 6 week-old healthy rice plants. After seven days symptoms were observed on the inoculated seedlings but not on control plants. The fungus was re-isolated and similar cultural characteristics were observed as of the original culture. DNA fingerprinting of the rDNA-ITS sequence of the representative isolate MNDSR01 (Gene Accession MH393598.1) matched with that of Sclerotium hydrophilum reference (MH393598) with 100% identity. To our knowledge, this is the first report of S. hydrophilum in the Cauvery command area of Karnataka.
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