First Report of Root Rot Caused by Corynespora cassiicola on Sesame in China

Sesame (Sesamum indicum L.) is one of the most important oilseed and food crops in China. In July 2015, sesame plants (variety Liaozhi No. 8) grown in a commercial field in Liaoyang, Liaoning Province, China, exhibited wilt and root rot symptoms. Disease incidence reached 25%. Typical disease symptoms included stunting, chlorosis of the lower leaves, and defoliation by the flowering stage. Reddish-brown, irregular lesions were observed on roots and feeder root density was reduced. As the disease progressed, roots turned violet-brown and developed rough skin with irregular longitudinal lesions and localized rots. Fruits of severely affected plants failed to mature. Symptomatic roots were collected, disinfested, and grown on potato dextrose agar (PDA). Cultures were obtained from single conidia incubated at 25°C for 10 to 15 days. Colonies were dense, villous, gray to dark brown, and often produced yellow or brown pigment on PDA. Conidiophores were brown, erect, broad at the base, formed singly or in groups, 1 to 20 septa, measuring 32 to 350 × 8 to 12 μm. Conidia varied in morphology. They were brown, cylindrical, curved, broad at the base, borne singly or in chains, 2 to 14 pseudosepta, measuring 30 to 255 × 6 to 15 μm. These characteristics are consistent with Corynespora cassiicola (Berk. & Curt.) Wei (Sato et al. 1980; Sinclair and Shurtleff 1975). The internal transcribed spacer (ITS) region of rDNA was amplified from the eight isolates using primers ITS1/ITS4 and sequenced. Same sequences were obtained and the sequence of isolate LSPC-1 was submitted to GenBank (accession no. MG569987). BLAST analysis of the sequence showed 100% similarity to C. cassiicola (KM107809). Pathogenicity test were conducted in a greenhouse with sesame variety Liaozhi No. 8. Inoculum was prepared from 12-day-old C. cassiicola cultures grown on PDA at 25°C. Conidia from the plates were suspended in distilled water and filtered through double layers of cheesecloth. Six 8-liter pots were filled with sterilized potting mix (equal parts sand, peat, and soil) and each pot was sown with 50 surface-sterilized sesame seeds. Before seeds were sown, sterilized potting mix in three pots was inoculated with 300 ml conidial suspension (10⁵ conidia/ml) respectively and thoroughly mixed by hand. The potting mix of three pots not inoculated with the conidial suspension served as controls. Plants were kept in a greenhouse until flowering stage with day temperatures of 26°C for 12 h and night temperatures of 24°C for 12 h. Symptoms developed 35 days after seedling emergence. Symptoms were similar to original disease observations, whereas no symptoms developed on the noninoculated controls. The same results were obtained when pathogenicity tests were repeated three times. C. cassiicola was isolated from inoculated plants. C. cassiicola has been reported as a root rot pathogen of soybean in Japan (Sato et al. 1980) and the U.S. (Raffel et al. 1999). In China, C. cassiicola causes target spot of soybean in Jilin Province (Chen et al. 1987) and leaf spot of sesame in Henan Province (Liu et al. 2015). To our knowledge, this is the first report of C. cassiicola causing root rot of sesame in China, and poses a threat to sesame production in China.