First Report of Rough Leaf Spot of Sorghum Caused by Ascochyta sorghi in China

Sorghum (Sorghum bicolor [L.] Moench) is an important food crop grown worldwide. In August 2015, long oval or irregular shaped and rough leaf lesions were observed on sorghum cultivar Liaoza 19 in Fuxin city of Liaoning Province and Chifeng city of Inner Mongolia, China. Early lesion development included small circular to oval spots with distinct margins. The center of the spots was lighter in color with a darker halo. Several lesions coalesced to form larger, distinct diseased areas on leaves. Numerous black, flask-shaped pycnidia were embedded on both sides of the lesion. The disease caused severe damage, killing leaves, and was most destructive on lower leaves. Disease incidence reached 65%. To identify the pathogen, symptomatic leaves were collected from fields and lesions excised and cut into small pieces. Leaf pieces were then surface disinfected in 2% NaClO for 2 min and rinsed with sterilized water three times. The small pieces of leaves containing lesions were dried with sterile filter paper and placed in Petri dishes containing water agar. The Petri dishes were placed in an incubator at 28°C for 2 to 3 days to allow pycnidiospore masses to exude from pycnidia. The causal organism was identified by pycnidiospore morphology and pycnidia produced on the lesions. Pycnidia were gregarious, globose, depressed, papillate, and variable in diameter, 135 to 246 μm. Pycnidiospores were hyaline, oblong elliptic, one-septate, constricted at the septum, and measuring 15 to 22.5 × 4.3 to 9.8 μm. Pure cultures on potato dextrose agar (PDA) were obtained from single pycnidiospore isolations. The colonies were white at first and grew slowly. Diameter of the colonies was 7 to 12 mm at 28°C for 15 days. Then dark brown pycnidia were produced on older colonies, and pycnidiospore masses exuded from pycnidia. These characteristics are similar to Ascochyta sorghi Sacc (Frederiksen et al. 1986). The internal transcribed spacer (ITS) region of rDNA was amplified from 12 isolates using primers ITS1/ITS4 (White et al. 1990), was sequenced, and that from isolate Cu-1 was submitted to GenBank (MG525547). Pathogenicity tests were conducted in the greenhouse with sorghum cultivar Liaoza 19 to fulfill Koch’s postulates. Inoculum was prepared from 15-day-old cultures of isolate Cu-1 grown on PDA at 28°C. Pycnidiospore masses were forced to exude pycnidia on PDA and suspended in distilled water with 3 × 10⁴ pycnidiospores/ml. All leaves on 42-day-old sorghum plants were sprayed with the pycnidiospore suspension, with a total of 16 plants inoculated. All leaves of another 16 pants were sprayed with distilled water and served as replicated, noninoculated controls. The inoculated plants were placed in an incubator at 28°C with 100% relative humidity for 48 h and then transferred to a greenhouse with day and night temperatures of 28 and 25°C, respectively. Ten days after inoculation, all inoculated plants exhibited light yellow, long oval or irregular shaped lesions with pycnidia forming inside the lesions 15 days after inoculation. All characteristics of the disease were similar to those observed in the original host. Noninoculated control plants remained asymptomatic. The same results were obtained when pathogenicity tests were repeated. A. sorghi was reisolated from inoculated plants and matched the morphological and molecular characteristics of the original isolates. No pathogen was reisolated from noninoculated plants. Rough leaf spot of sorghum was first reported in Italy by Saccardo (1878). In China, rough leaf spot was first observed on sudangrass in Nanjing city, Jiangsu province, in 1955 (Liu et al. 1955). Since 1955 there have been no reports of this disease on sorghum. This is the first report of A. sorghi causing sorghum rough leaf spot in China in the modern era. So far, we have observed the disease on sorghum in Inner Mongolia, Liaoning, and Shandong Province. It has become an important leaf disease of sorghum in recent years, and it may pose a threat to sorghum production in China.