First Report of Sorghum Zonate Leaf Spot Caused by Gloeocercospora sorghi in China

Sorghum [Sorghum bicolor (L.) Moench] is not only an important food crop, but is also used as forage, brewing, and industrial raw materials in China. In October 2016, large purple-red, irregular lesions were observed on leaves of several sorghum varieties in Chaoyang, Liaoning Province, China. The most serious symptom occurred on sorghum variety Jiza140 and the yield losses were ∼30%. Lesions on leaves were purple-red or dark brown, nearly circular or irregular, with 2 to 8 obvious rings. Semi-oval shaped lesions occur along the leaf margin or near the midrib. In severe cases, lesions join to cover a large portion of the leaf surface. To identify the pathogen, symptomatic lesions were collected from fields and cut into small pieces, surface-sterilized in 75% ethanol for 5 s, then disinfested in 2% NaOCl for 2 min and rinsed with sterilized water. The small pieces of leaves containing lesions were placed on water agar (WA) at 25°C to allow sporulation. After incubation for 4 to 5 days, readily visible pink to salmon-colored gelatinous spore masses with typical conidia were produced on lesion tissue. The conidiophores were short (5 to 11 µm), hyaline, and either simple or branched, densely clustered. Pure cultures on V-8 agar were obtained from single conidia. Colonies were white to gray and round, aerial mycelium was short, and colony margins were irregular. Conidia were born in a pinkish to salmon-colored slimy matrix, hyaline needle-shaped, straight or slightly curved, tapering somewhat from the base to the apex, 3 to 17 septations, 28 to 197 × 1.6 to 3.4 µm (avg. 15.9 × 2.2 µm). These characteristics are similar to Gloeocercospora sorghi Bain & Edgerton (Frederiksen et al. 1986). The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS1/ITS4 and sequenced. Same sequences were obtained and the sequence of isolate BW1 was submitted to GenBank (accession no. KY862005). BLAST analysis of the sequence showed 100% confirmation to G. sorghi (LC063855). Pathogenicity was tested in greenhouse with sorghum variety Jiza140 to fulfill Koch’s postulates. Inoculum was prepared from 7-day-old cultures of G. sorghi grown on V-8 juice agar under longwave UV at 24°C. Conidial masses were the predominant fungal structure formed. Conidia from the plates were suspended in distilled water with 10⁶ conidia/ml. Leaves of 35-day-old sorghum plants were sprayed with the conidial suspension, and 10 plants were inoculated. Leaves sprayed with distilled water were the noninoculated control. The inoculated plants were placed in an incubator set at 28°C with 100% RH for 24 h and transferred to a greenhouse with day and night temperatures of 28 and 24°C, respectively. Two days after inoculation, all inoculated leaves of plants had small, chlorotic, water-soaked lesions. Five days after inoculation, all inoculated plants showed characteristic lesions on leaves, while no symptoms developed on noninoculated controls. The same results were obtained when pathogenicity tests were repeated twice. G. sorghi was reisolated from inoculated plants and matched the morphological and molecular characteristics of the original isolates, but the pathogen was not reisolated from noninoculated plants. G. sorghi was first reported as a pathogen of sweet sorghum in Louisiana (Williams et al. 1978). In China, it was first isolated from sorghum leaf samples collected from Jilin Province in 1966, but was not reported as a causal agent of the disease (Qi 1978). To our knowledge, this is the first report of G. sorghi causing sorghum zonate leaf spot in China, and it poses a threat to sorghum production in China.