Indian Phytopath. 64 (1) : 41-43 (2011) Stem rot of rice caused by Sclerotium oryzae Catt. (= Magnaporthe salvinii (Catt.) Krause and Webster) is one of the major constraints for rice production in the Indian subcontinent especially in Haryana (Singh et al ., 2002). The disease affects the stem resulting in rotting and subsequent lodging of the crop. The pathogen has been reported to cause substantial losses in grain yield ranging from 5-80 per cent (Kumar et al., 2003). Since resistance to stem rot in rice genotypes is rare (Raina et al ., 1980; Chand et al ., 1985) and for applying efficient strategies for developing resistant genotypes through breeding process, the knowledge about the presence of natural populations of pathogen is pre-requisite. The present investigation highlights the variability within S. oryzae isolates collected from four major rice growing areas of Haryana and proposes a set of differential hosts for differentiating virulence in the pathogen populations. MATERIALS AND METHODS Cultural and morphological variability The cultures were isolated from the stem rot infected samples collected from different cultivars of rice grown in various localities of Haryana. The cultures were purified by transferring apical growth of fungal colonies from 2% water agar to potato dextrose agar slants. The Petriplates were inoculated with 5 mm mycelial disc obtained from the periphery of actively growing colony of five days old cultures of each of the fifteen isolates (designated as SO 1 – SO 15) and incubated at 25±2°C. The colony diameter was recorded 72 h after incubation while sclerotial characters were recorded at 24 h intervals up to 10 days. The sclerotial size was observed by measuring 100 sclerotia from each Petriplate using ocular and stage micrometer. Colony growth was determined by taking average of diametric growth measured at right angles to each other. Pathogenic variability Ten rice genotypes viz. BR 51-282-8-HR 45, BRC 16-127- 4-1, CR 689-116, CR 837, Govind, HKR 126, IR 48725-B- B-103-2-3, Jaya, Pusa 33 and PR 106 were used to study the pathogenic variability among S. oryzae isolates. Thirty days old seedlings of the genotypes were transplanted in triplicate set of 5-m long rows with a spacing of 15 cm between rows and plants. The inocula of 15 test isolates of the pathogen were multiplied in Petriplates containing 30 ml of sterilised PDA at 25±2°C for 15 days. Both actively growing mycelium along with agar and sclerotia were used for inoculation. Fifty rice tillers, comprising of lower 2-3 nodes of each genotype, were detached at flowering stage. These stem pieces were punctured with needle in the centre of stem internode and inoculated with mycelial bit along with sclerotia of each of the fungal isolates in laboratory following cut wound inoculation method for two consecutive seasons. The inoculated stem pieces were incubated at 25±2°C. The genotypes showing a lesion length of <10 mm were designated as resistant and >10 mm as susceptible (Hseih, 1966). RESULTS AND DISCUSSION Cultural and morphological variability All the fifteen isolates showed marked differences in their growth, sclerotial characters on potato dextrose agar. These isolates of S. oryzae could be categorised into three groups. Group 1 had the fastest growing isolate SO 2 with a mycelial diameter of 90 mm after 72 h of incubation with sclerotia *Corresponding author: singhdrram@yahoo.co.in Variability in Sclerotium oryzae isolates causing stem rot of rice based on cultural, morphological and pathogenic behaviour from Haryana regions A. KUMAR, RAM SINGH*, K.S. BOORA 2 and B.L. JALALI Department of Plant Pathology, 2 Department of Molecular Biology and Biotechnology, CCS Haryana Agricultural University, Hisar 125 004 *CCS Haryana Agricultural University, Rice Research Station, Kaul 136 021 ABSTRACT: Fifteen isolates of rice stem rot fungus Sclerotium oryzae Catt. (SO1 – SO15) were examined for their variability. These isolates were collected from major cultivars and locations of Haryana state. The isolates exhibited variation in their growth, sclerotial position and pattern on potato dextrose agar. Out of ten, six rice genotypes viz. BR 51-282-8-HR 45, BRC 16- 127-4-1, CR 689-116, CR 837, HKR 126 and PR 106 showed differential response against the pathogen. On the basis of pathogenic behaviour, the isolates were categorised into four pathogenic groups. Accordingly, these six rice genotypes could be used as differential hosts for study of host-pathogen interaction for an efficient rice resistant breeding programme against stem rot. Key words: Pathogenic variability, rice genotypes, resistance, Sclerotium oryzae, stem rot, virulence RESEARCH ARTICLE