Study On Sorghum Sudanense Leaf Spot Caused By Bipolaris Sorghicola

Sorghum sudanense leaf spot is considered as a widespread disease of sorghum sudanense in south China. This paper systematically studied two aspects:one is on the occurrence and symptoms of the disease, and the other is on the identification, biological characteristics, the promoting sporulation conditions and infection conditions of the pathogen. The main results are as follows.1. Symptom and pathogen identification of sorghum sudanense leaf spotSorghum sudanense leaf spot mainly appears on the leaves. The early appearance of its symptom is small watery spots, and then spots gradually expand, with the central changing into colorless and the edge having remained dark. The typical leaf spot was dark purple-red color, nearly oblong. At the later period of disease, spots extend together into pieces, and the leaves caused necrosis.According to morphological and molecular identification, the pathogen of sorghum sudanense leaf spot was identified as imperfect fungi, Bipolaris sorghicola. The conidiospore of Bipolaris sorghicola was nearly cylindrical or fusiform, often slightly curved, the central width, both ends of the relatively narrow, size 41.8～82.5μm×12.7～16.5 (61.6×14.2)μm, and with 5 to 9 diaphragms (mostly 7).2. Biological characteristics of Bipolaris sorghicolaIt showed that for mycelium growth, the temperature was 15～40℃and the pH was 4-10. The optimum temperature and pH was 25℃and 8, respectively. All kinds of carbon and nitrogen sources could be effectively used by pathogen, especially glucose, lactose, soluble starch, glycine, proline, and peptone. The mycelium could grow on different media. Oatmeal medium was the best medium for the pathogen. The conidiospore of Bipolaris sorghicola could germinate at 15～40℃and under pH 4～9, while the optimum temperature and pH was 25℃and 6, respectively. Constant light promoted conidiospore germination slightly, but not sensitive. The lethal temperature of mycelium and conidiospore exceeded 50℃ 3. Sporulation conditions of Bipolaris sorghicolaBipolaris sorghicola rarely produce conidiospore in the regular culture conditions. To investigate sporulation conditions by changing the culture conditions, pathogen on Tomato juice medium sporulated the most, and then corn meal medium. Soluble starch and peptone slightly promoted the sporulation. The plant material medium promoted the sporulation more effectively than that the chemical reagents medium did. Light had no significant effect on its sporulation, but the appropriate time of UV irradiation could promote sporulation. The pathogen sporulated the most at 25℃.4. Infection conditions of Bipolaris sorghicolaThe infection of sorghum sudanense leaf spot by Bipolaris sorghicola could occur at 15～40℃. It became serious when the temperature exceeded 30℃. The disease became more serious along with the temperature going up. The wetness period for infecting leaves was 12～60 h. It became serious when the wetness period exceeded 36 h. The disease became more serious with the wetness period rising. Pathogens could not infect sorghum sudanense by seeds and roots. The wounded leaves and old leaves were prone to be infected. Overall, the experiment indicated that the Bipolaris sorghicola had very short incubation period and strong pathogenicity.