First Report of Stem Rot on Hosta plantaginea caused by Sclerotium rolfsii in Korea

Abstract

A stem rot disease caused by Sclerotium rolfsii sporadically afflicted fragrant plantain lily (Hosta plantaginea Aschers.) from 2008 to 2010 in the herb exhibition garden at Gangju Pond, which is located in Jeongchon-myon, Jinju, Korea. The fragrant plantain lilies, which are perennial plants grown in home gardens, recently were planted en mass in parks and public gardens for ornamental purposes, where they form dense communities. During summer, the stems of the plants under the canopy are exposed to warm temperatures and high humidity, which favor disease development. The disease developed mainly on stems near the soil line. Infected stems gradually died, and white mycelial mats appeared on the stem surfaces (Fig. 1A). Often, numerous sclerotia were produced on stem surfaces near the soil line. The heavily infected stems became rotted and blighted, and the whole plant eventually died. The freshly isolated pathogenic fungus grown on potato dextrose agar (PDA) was examined microscopically and culturally. The optimal growth temperature on PDA was 30°C. Aerial mycelia usually formed many narrow hyphal strands 3-9 μm wide. The white mycelium had a typical clamp connection structure for 4 days after growth at optimum temperature (Fig. 1D). Small globoid sclerotia formed abundantly on the PDA after 20 days of mycelial growth (Fig. 1C). The sclerotia, which were relatively uniform in size at 1-3 mm, were white at first, and then turned dark brown. The maximum number of sclerotia was produced at 25-30 oC. To test pathogenicity, inoculums were prepared with mycelial mats and mixed with autoclaved soil. Then, soil inoculums containing mycelial mats were artificially inoculated onto Hosta plantaginea. The same fungal fruiting symptoms were reproduced, and the fungus was re-isolated based on these symptoms (Fig. 1B). To identify the causal fungus, we amplified and sequenced a partial internal transcribed spacer (ITS) rDNA region of the isolate using the primers ITS1 and ITS4, as described by White et al. (1990). The resulting sequence of 684-bp was deposited in GenBank (accession no. HM355751). Phylogenetic analysis was performed using MEGA4 software, with the neighbor-joining method and Tajima–Nei distance model. Previously published ITS sequences of S. rolfsii strains were included for reference, and S. cepivorum was used as the outgroup (Harlton et al., 1995). Neighbor-joining analysis showed that the two species had similar ITS sequences (Fig. 2). In the phylogenetic tree (Fig. 2), the isolate was placed within a clade comprising reference isolates of S. rolfsii.