First report of Rhizoctonia solani AG-1 IA causing rice sheath blight of Oryza rufipogon in China.

Red rice (Oryza rufipogon Griff.) is a valuable source of important agronomic traits as well as genes for biotic and abiotic stress tolerance. In June 2020, rice sheath blight on O. rufipogon cv. Bin09 was observed in Zhanjiang (20.93N, 109.79E), China. Initial symptoms on sheaths were water-soaked and light green lesions. Then, the lesions gradually expanded into oval or cloud shaped lesions with a gray white center. The lesions coalesced, causing the entire sheath to become blighted. Disease incidence reached approximately 30% in the fields (10 ha) surveyed. Twenty sheaths with symptoms were collected and cut into pieces of 2 × 2 cm in size. They were surface-disinfected with 75% ethanol for 30 s and 2% sodium hypochlorite (NaOCl) for 60 s, rinsed three times with sterile water, blotted dry on sterile paper, plated on potato dextrose agar (PDA), and incubated at 28°C in the dark for 4 days. Thirty-six pure cultures were obtained by transferring hyphal tips to new PDA plates, and three isolates (ORRS-1, ORRS-2, and ORRS-3) with similar morphological characteristics on PDA were selected as the representative isolates for study. Colony of isolate ORRS-1 was white initially, then turned brown with brown sclerotia. Septate hyphae were hyaline, smooth, and branched at right angles with a septum near the point of branching. Based on these morphological characteristics, the fungus was identified as Rhizoctonia solani Kuhn (Sneh et al. 1991). The isolates were deposited in the fungus collection of the Aquatic Organisms Museum of Guangdong Ocean University. For molecular identification, genomic DNA from each of the three isolates was extracted, and the internal transcribed spacer (ITS) region was amplified, and sequenced with the primer pair ITS5/ITS4 (White et al. 1990). The sequences were deposited in GenBank (accession nos. OP497977 to OP497979). The three isolates were 100% identical (716/716 bp; 716/716 bp; and 716/716 bp) with those of R. solani AG-1 IA (accession nos. KX674518, MK481078, and MK480532) through BLAST analysis. The phylogenetic tree grouped the three isolates within the R. solani AG-1 IA clade with high bootstrap support (99%) by the maximum likelihood method. A pathogenicity test was performed with these three isolates in a greenhouse at 24 to 30°C. Approximately 50 seedling of red rice cv. Bin09 were grown in each cup ( 250 ml in size with sterile soil 50 cm3). At the 3-leaf stage, plants in five cups were inoculated with each isolate by spraying a mycelial suspension (106 mycelial fragments/ml) until runoff. The mycelial suspension was prepared by adding sterile distilled water to the cultures and gently scraping the surface with a sterilized scalpel blade. Five plants sprayed with sterile water served as the controls. The test was conducted three times. Sheath blight was observed on the inoculated leaves after 15 days while no disease was observed in the control plants. Morphological characteristics and the ITS sequences of fungal isolates re-isolated from the diseased sheaths were identical to those of R. solani AG-1 IA. R. solani AG-1 IA is one of the most important plant pathogens worldwide, causing foliar diseases on maize, rice (O. sativa L.), and soybean (Joana et al. 2009). To our knowledge, this is the first report of R. solani AG-1 IA causing rice sheath blight of O. rufipogon in China (Farr and Rossman, 2022). With the spread of the pathogen on weedy populations of red rice, resistant races or pathotypes may evolve that could spread to cultivated rice.