First report of Sclerotium hydrophilum on leaf sheath of rice (Oryza sativa ) in south-eastern Australia

Abstract

Aggregate sheath spot caused by Rhizoctonia oryzae-sativae, and sheath spot caused by Waitea circinata have recently been reported on rice in Australia (Lanoiselet et al., 2001). Several disease surveys were conducted to investigate the presence of other sclerotial diseases of rice. In 2001 and 2002, infected leaf sheaths were observed on several rice plants. Pieces of infected leaf sheaths were surface-sterilized for 2 min with sodium hypochlorite solution diluted to give 2% v/v available chlorine, rinsed with sterile distilled water, placed on potato dextrose agar (PDA) and incubated at 25°C. Eighteen isolates of R. oryzae-sativae, 10 isolates of W. circinata and two isolates of an unknown fungus were isolated. The unknown fungus was identified by the Centraalbureau voor Schimmelcultures (Utrecht, the Netherlands) as Sclerotium hydrophilum (NSW Agriculture accession number DAR 75575). In 2002, soil and floating plant material were collected from rice fields and screened for the presence of sclerotia. Tiny globose black sclerotia were recovered from five samples out of six. These sclerotia germinated on PDA and the sclerotia that subsequently formed were identified as those of S. hydrophilum. Koch's postulates was completed by inoculating rice plants with plugs of PDA bearing both mycelium and sclerotia of S. hydrophilum. After 10 days at 30°C, sheath leaf necrosis was observed around the inoculation plugs. S. hydrophilum was re-isolated from the lesions. Several authors have previously reported S. hydrophilum as a pathogen of cultivated rice (Cedeno et al., 1997). Kernkamp et al. (1977) reported S. hydrophilum on wild rice (Zizania aquatica) and Johnson et al. (1976) on water lilies (Nymphaea odorata). This is the first report of S. hydrophilum occurring on cultivated rice in Australia. This work was funded by the Cooperative Research Centre for Sustainable Rice Production.