Stem rot of rice caused by Sclerotium hydrophilum isolated in Myanmar

Rice sheath and stem diseases caused by Rhizoctonia and Sclerotium species are problems in rice growing regions of Southeast Asia. In the summer season of 2007, in rice fields of PyinmanaTownship in Myanmar, rice stems were visually observed with brownish to black lesions without distinct margins (Fig. 1). The causal pathogen was isolated by placing 1 cm diseased stems on 2% water agar media and incubating at 28°C for seven days. The hyphae of the non-sporulating culture was stained with a mixture of safranine solution and 3% KOH (Bandoni, 1979). The hyphal width was measured to be within a range of 6 to 10 µm and binucleate hyphae were observed between the septa (Fig. 2). Small, globose and reddish brown to black coloured mature sclerotia measuring 0.24 - 0.43 mm were observed in culture (Fig. 3). Based upon these characteristics, the fungus was initially identified as Sclerotium hydrophilum.On comparing the rDNA-ITS sequences of the isolate (GenBank Accession No. FJ595944) with that of a S. hydrophilum reference (DQ875597) there was found to be 99% identity (Fig. 4).

Figure 1: Symptom expression of rice stem naturally infected by S. hydrophilum

Figure 2: Hyphae of S. hydrophilum observed under light microscope. The arrows indicate nucleus (a) and septum (b)

Figure 3: Culture of S. hydrophilum showing sclerotia

To confirm pathogenicity, ten rice sheath tissues were inoculated by placing a five day-old mycelial plug on the stem 1 cm below the axil of the fully mature leaf and wrapping with parafilm.Control plants were treated in the same manner using a plug of 2% water agar. After five days, symptoms on all inoculated plants (Fig. 5) were similar to those occurring in nature.Control plants did not develop symptoms. The fungus was re-isolated from inoculated plants, confirming Koch's postulates.

Figure 4: Sequence alignment of isolate (msh1) and S. hydrophilum reference (DQ875597)

Figure 5: Symptom expression of rice stem inoculated with S. hydrophilum

Stem rot of rice caused by S. hydrophilum has been reported previously in other rice growing countries in Southeast Asia (Kimiharu et al., 2004), but this is the first report of this pathogen on rice from Myanmar. Further surveys and isolate collection will be needed to evaluate its geographic distribution and prevalence in Myanmar.