First report of basal stem rot on sugarcane (var. Badila) caused by Sclerotium rolfsii in China.

Badila (Saccharum officinarum) is one of the important chewing cane in south China. During the year 2019-2020, as much as 60.2%-87.5% of sugarcane plants stem showed red rot developments were observed in the fields of Yongning District, Nanning city, Guangxi province. Symptomatic plants showed red rot at basal stem nodes and sheath, when the disease serious, the epidermis and aerial roots decomposed and exfoliated, then formed sclerotiums, the upper stem also occurred the symptom. Infected plant tissues were dissected into small pieces with 0.1 × 0.1cm in size and surface sterilized in 0.1% HCl2 for 2 min, followed by 75% ethanol for 30 s, rinsed three times with sterile distilled water. Then the tissues were placed onto potato dextrose agar (PDA) plates and incubated at 25 °C for 3 days. Numerous white globoid sclerotia were formed on PDA after 5 days of growth. The sclerotia (2 to 3 mm in diameter) were white at first and then gradually turned dark brown. Aerial mycelia usually formed many narrow hyphal strands 4 to 9 μm wide. Five uniform isolates were obtained from diseased sugarcane plants. Pathogenicity of representative strain W1 was confirmed by inoculating 120-day-old Badila plants grown in field. Five plants were inoculated with colonized agar discs (6mm in diameter) by applying toothpick tips to the lower part of the stem. Five non-inoculated plants served as control. The inoculated and non-inoculated plants were sprayed sterile water then incubated with plastic film for maintained high moisture. All the plants were placed inside of a growth chamber at 26 ± 2°C with a 14-h photoperiod and 80% relative humidity. All inoculated plants showed red rot at stem and sheath after 2 weeks, whereas the control plants were symptomless. By the third week, mycelium and sclerotia developed on the crown on the inoculated plants. The fungus was re-isolated from the artificially inoculated plants. To confirm the species-level identification, partial of the ribosomal DNA internal transcribed spacer (ITS), mitocondrial small subunit (SSU), and nuclear ribosomal large subunit (LSU) regions of representative strain W1 were amplified and sequenced using the primers pairs ITS1/ITS4 (White et al. 1990), ITS-Fu-F /ITS-Fu-R and SRLSU1//SRLSU2 (Kumar et al., 2016), respectively. The resulting ITS, SSU and LSU sequences were deposited in GenBank (GenBank accession no. MW620994, MW617878, and MW617872) and shared 99.42%, 100% and 100% sequence identity with Athelia rolfsii isolate (JN017199, OM319631, and MT225781). Phylogenetic analysis conducted with neighbor-joining (NJ) method using MEGA6.0 revealed that the isolate share a common clade with reference sequence of A. rolfsii in GenBank Data Library. Based on morphological and molecular characteristics, the fungus was identified as A. rolfsii (anamorph: Sclerotium rolfsii) (Paul et al. 2017; Paparu et al. 2020). Although S. rolfsii has been reported causing sugarcane sett rot in Australia (Bhuiyan et al., 2019) and seedlings of sugarcane in Indian (Gopi et al., 2023), as we know, this is the first report of sugarcane basal stem rot disease caused by this fungus in China. This study will be helpful for the prevention and control sugarcane basal stem rot in the future.