First Report of Fusarium oxysporum Causing Root Rot in Ophiopogon bodinieri in China

Abstract

Ophiopogon bodinieri H. Lev. is an important ornamental groundcover widely used in urban gardens in southern China (Liu et al. 2011). In September 2017, a disease occurred on approximately 20% of O. bodinieri in 9 ha in Guangzhou, Guangdong province in China. Symptoms included etiolation in the leaves, wilt, root rot, and necrotic vascular systems. Three diseased plants were sampled for pathogen isolation. Portions (about 5 mm²) of symptomatic root tissues were dissected and surface disinfected (3% NaClO for 10 s and 70% ethanol for 30 s). Tissues were rinsed three times using sterile distilled water, dried on sterile filter paper, and transferred to Petri plates with potato dextrose agar (PDA) supplemented with streptomycin sulfate (150 µg/ml). Petri plates were incubated at 28°C for 5 days (Dita et al. 2010). Only one isolate was obtained from all the plates and was subcultured to new PDA plates. A single-spore isolate was obtained from a hyphal tip, and the culture characteristics and conidial morphology were studied on PDA and carnation leaf agar (CLA) (Neish 1983). The isolate grown on PDA formed abundant white-colored fungal colonies with radial mycelium in 5 days at 28°C. Microscopic observations from CLA medium revealed the curved macroconidia were usually three- to five-septate, with the size of 2.2 to 4.0 × 18.3 to 42.4 μm. Microconidia were kidney shaped with the size of 4.7 to 6.8 × 7.3 to 12.1 μm. Chlamydospores were single or in clusters, with the size of 9.1 to 11.0 μm in diameter. The elongation factor 1-alpha (EF1α) gene (accession no. MN026924, 686 bp), amplified and sequenced using primer pair EF-1/EF-2 (O’Donnell et al. 1998), showed 100% identification to a Fusarium oxysporum strain (accession no. KY508353.1) (Geiser et al. 2004). The molecular identification was confirmed via BLAST on the Fusarium ID and Fusarium MLST databases. Ten-week-old plants were used for pathogenicity tests. First, the plants were wounded by cutting off 1 cm of the roots. Then, 10 plants were inoculated by root dipping (30 min, 10⁴ spores/ml), and another 10 plants were treated with sterile water as a control. The plants were then repotted in potting mix and incubated at 28°C. The assay was conducted three times. After 15 days, the plants showed symptoms of leaf wilting, root rot, and necrosis in vascular tissues. After 40 days, all the inoculated plants were dead, whereas no symptoms were observed in the controls. Subsequently, the F. oxysporum isolate was successfully reisolated from the inoculated plants and was identified again by sequencing the EF1α. The pathogen was further identified by PCR amplification and sequencing the internal transcribed spacer (ITS) gene region using the primers ITS5/ITS4 and the 18s nuclear ribosomal small subunit (SSU) using the primers NS1/NS4 (Schoch et al. 2012). The isolate showed 100% and 99% identity to those of F. oxysporum (accession KF498869.1 for ITS and KU512835.1 for SSU). The sequences of ITS (accession no. MH752745.1), SSU (accession no. MH752591.1), and EF1α (accession no. MN026924) were deposited in GenBank. The first F. oxysporum causing disease on another Ophiopogon species (O. japonicus) was discovered in Florida in 1991 (Farr and Rossman 2019). To our knowledge, this is the first report of root rot disease of O. bodinieri caused by F. oxysporum in the world. This disease may pose a risk for urban landscapes in China.