First report of soybean root rot caused by Fusarium proliferatum in the Republic of Korea.

Fusarium species are widespread soilborne pathogens that can cause damping-off, root rot, and wilting in soybean [ Glycine max (L.) Merrill], subsequently leading to significant yield suppression. Several Fusarium spp. have already been documented for their pathogenicity on soybean plants in the Republic of Korea. The nationwide monitoring of soybean diseases continues to identify new pathogenic Fusarium spp. In 2016, five plant samples at R3-R4 growth stages, showing symptoms of wilting in the upper parts and root rot, were collected in Suwon, Gyeonggi, Republic of Korea. Fungal colonies were obtained from the diseased root samples, with the surface sterilized in 1% sodium hypochlorite for 2 min, rinsed thrice with sterile distilled water, and placed on water agar at 25°C. Five isolates were collected and purified by single-spore isolation. The fungal mycelium was subsequently cultivated on potato dextrose agar for ten days. The isolates produced abundant, aerial, and white mycelium and became purple in old cultures. Macroconidia were slender, falcate to almost straight, usually 3 to 5 septated, and thin-walled. Microconidia were formed in chains from polyphalides, clavate or oval, usually single-celled with a flattened base. These characteristics of isolates were consistent with the description of F. proliferatum (Leslie and Summerrell 2006), and the representative isolate 16-19 was selected for molecular identification to confirm its identity as F. proliferatum . Two evolutionarily conserved genes, the translation elongation factor 1-alpha ( EF-1α ) and the second-largest subunit of RNA polymerase II ( RPB2 ) genes, were partially amplified using the primers described by O'Donnell et al. (2008), resulting in nucleotide sequences of 680 and 382 base pairs, respectively. These two sequences (GenBank accession numbers: OQ992720 and OR060666) showed 100 and 99.5% identity to the EF-1α and RPB2 of F. proliferatum A40 (GenBank accession numbers: KP964907 and KP964842). For the Petri-dish pathogenicity assay (Broders et al. 2007), five surface-sterilized seeds were placed on water agar media with either sterile water or actively growing '16-19' culture. After 7 days of incubation in a growth chamber (25°C; 12-hour photoperiod), brown lesions were observed on the roots of the inoculated plants, while no symptoms were observed in the sterile water-treated controls. The experiment was conducted three times. For root-cut pathogenicity assay, conidial suspension (1×106 conidia/ml) of the isolate '16-19' was prepared with harvested mycelia cultured on PDA for 10 days with sterile water. The roots of 10-day-old soybean seedlings were partially cut and soaked in either the suspension or sterile water for 2 hours. The seedlings were transplanted into 12 cm plastic pots (11 cm in height) and grew in a greenhouse (26 ± 3°C, 13-h photoperiod). The experiment followed a completely randomized design with three replicates (i.e. three plants in a pot), and it was repeated twice. The inoculated plants began to wilt 7 days after inoculation, while the sterile water-treated controls remained healthy. Ten days after inoculation, all plants were collected, washed under running tap water, and evaluated for the presence and severity of root rot using a 0-4 scale (Chang et al. 2015). The inoculated plants exhibited reduced vigor and developed dark brown lesions on their roots. F. proliferatum was reisolated from symptomatic root tissues of the infected plants, while not from those of the controls. Its colony and spores were morphologically identical to those of the original isolate. F. proliferatum was previously reported as a causative agent of soybean root rot in the United States (Díaz Arias et al. 2011) and Canada (Chang et al. 2015). This is the first report of soybean root rot caused by F. proliferatum in the Republic of Korea. This finding implies that F. proliferatum may potentially threaten soybean production in the Republic of Korea and suggests that effective disease management strategies should be established for soybean protection against the disease, along with continuous surveillance.