First Report of Sclerotium hydrophilum Causing Stem Rot of Rice in Northeast China

Abstract

In August 2016, rice plants with brown to black lesions with irregular shape and water-soaked margins in leaf sheaths were observed in Baoqing county in Heilongjiang province, Northeast China. To isolate the causal pathogen, pieces of symptomatic leaf sheaths were treated with 1% NaClO for 1 min, rinsed with sterile distilled water for 2 min, and then transferred to potato dextrose agar (PDA) medium for incubation at 28°C. The cultivated mycelium was transferred to new PDA medium. Colonies of pure cultures on new PDA medium were initially white and turned brown about 3 weeks later. The hyphal width was measured with a range of 4.5 to 6.2 μm. Large numbers of small and globose sclerotia were observed on surface of the colonies at 3 days after subculturing. The sclerotia were white at first and then turned black over time. The diameters of sclerotia ranged from 0.31 to 0.54 mm with an average of 0.41 mm (n = 50). DNA of a representative isolate named Hbq001 was extracted, and the 18S rDNA region was amplified by PCR with universal primer pair NS1/NS6 (White et al. 1990). The 18S rDNA sequence of isolate Hbq001 (GenBank accession no. KY995575) shared 99.7% nucleotide identities with those of Sclerotium hydrophilum (GenBank accession no. KC354147). Therefore, based on these cultural characteristics and rDNA sequence, isolate Hbq001 was identified as S. hydrophilum (Demirci et al. 2009). To complete Koch’s postulates, 4 mm diameter PDA disks containing mycelium and sclerotia of S. hydrophilum isolate Hbq001 were placed on the stem of 4-week-old healthy rice seedlings and wrapped with Parafilm. Sterile PDA disks were used as controls. After 4 days at 28°C, symptoms similar to those in the field were observed on the inoculated seedlings but not on control plants. Finally, the fungus S. hydrophilum, which showed the same cultural characteristics as described above, was reisolated from the inoculated seedlings. In China, S. hydrophilum was first identified in Yunnan province and was found to be distributed in southern China (Hu et al. 2008; Ruan and Chen 2002; Wang et al. 2013). Northeast China is a major rice-producing region. Differing from the rice-producing areas of southern China, this region has specific climatic conditions with cold temperature and abounds in high-quality japonica rice. To our knowledge, this is the first report of S. hydrophilum in the cold northern region of China.