Sugarcane (Saccharum officinarum L.) is an important sugar crop widely cultivated around the world. Guangdong is the third largest sugarcane production province in China. In December 2024, a new red stripe disease was observed on sugarcane in a breeding field of approximately two hectares at South China Agricultural University (23°09′N, 113°22′E), with an incidence of 5% to 8%. The disease initially appeared as small red stripes, which elongated longitudinally, eventually coalescing into lesions with irregular margins. Symptomatic leaf segments (approx. 0.5 cm) were excised from the junction of diseased and healthy tissue. The segments were surface-sterilized by immersion in 75% ethanol for 30 seconds, followed by treatment with 0.1% HgCl₂ for 3 min. After being rinsed three times with sterile distilled water and dried on sterile filter paper, the segments were placed on PDA plates and incubated at 25 °C in complete darkness. Nine fungal strains were isolated by monosporic culture from three leaf samples. Seven of these strains exhibited similar morphological characteristics and were therefore selected for in vitro pathogenicity assays on detached sugarcane leaves. Following wounding, the leaves were inoculated with each of the seven strains and maintained at 28 °C and 80% relative humidity for six days. Strain PL2414 was determined to be the most virulent among them. The colonies of isolate PL2414 were circular, light olive-green, with a well-defined white tomentose margin and a yellowish-brown reverse. The conidia were obclavate to ovoid, measuring 20.2 to 33.1 × 13.6 to 16.9 μm (n = 50), with two to five transverse septa, and were either beakless or possessed a short beak. Morphologically, the isolates were identified as Alternaria spp. (Guo et al. 2024). Molecular identification was performed based on partial sequences of ITS (ITS4 and ITS5 primers) (White et al. 1990), TEF1-α (EF1-728F and EF1-986R primers), GAPDH (GPD1 and GPD2 primers), Alt a 1 (Alt-al-for and Alt-al-rev primers) (Hong et al. 2005) and RPB2 (RPB2-5 and RPB2-7c primers) (Li et al. 2022) genes. The sequences were deposited in GenBank (ITS: PX048794, TEF1-α: PX088168, GAPDH: PX088158, Alt a 1: PX088156, RPB2: PX088166). A BLASTn analysis revealed that the obtained sequences exhibited 99% nucleotide sequence identity to Alternaria gossypina AB7-4. Therefore, the isolate PL2414 was identified as A. gossypina through a combination of morphological and molecular characteristics. For pathogenicity tests, leaves of six healthy potted sugarcane seedlings (five- to six-leaf stage) were wounded with a sterile needle and inoculated until runoff with a conidial suspension (10⁶ conidia/mL). Control plants were treated with sterile distilled water. All plants were maintained in a greenhouse at 28 °C with 80% to 90% relative humidity. At 14 days post-inoculation, no symptoms were observed on control plants, whereas all inoculated seedlings developed red stripes identical to those observed in the field. Koch's postulates were completed by re-isolating A. gossypina from symptomatic leaves. To our knowledge, this is the first report of A. gossypina causing sugarcane red stripe disease in China, which serves as a basis for the control of this disease.
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