Occurrence of <i>Colletotrichum siamense</i> Causing Ring Spot on Sugarcane in China

Sugarcane (Saccharum officinarum L.) is an important sugar and economic crop, mainly cultivated in tropical and subtropical regions, where it plays a key role in the local economy. Its stems can be processed into sugar, ethanol, and feed. In October 2025, a severe leaf spot outbreak (disease incidence &gt; 50%) was observed at the sugarcane research institute of Guangxi academy of agricultural sciences of China (22°50′58″N, 108°14′47″E), covering an area of approximately 150 m2. Initially, small brown or reddish-brown spots appeared on the leaves, which later expanded into elliptical or irregular lesions. In severe infections, these lesions coalesced, causing the leaves to wither, yellow, and senesce prematurely. Leaf samples with leaf spot symptoms were collected from infected sugarcane plants for pathogen isolation. Leaf tissues (5 mm²) were excised from the lesion margins, surface-sterilized (75% ethanol, 30 s; 2% NaClO, 2 min), rinsed five times in sterile water, and placed on potato dextrose agar (PDA), followed by incubation in the dark at 28°C. Three morphologically distinct isolates were obtained by transferring hyphal tips to fresh PDA medium. One isolate was preliminarily identified as Colletotrichum siamense (designated Jiang3.2, a new host record) based on morphology and ITS sequencing, and was selected for further characterization. The isolate on PDA formed white to grayish-white colonies with dense aerial mycelium and dark brown to black pigmentation on the reverse. Conidia were unicellular, hyaline, straight, cylindrical with bluntly rounded ends, measuring (12.6-17.8) × (3.4-6.2) μm (n = 100). Hyphae were hyaline and septate. Appressoria were melanized, oval to clavate in shape, irregular, measuring (10.1-15.0) × (5.1-7.4) μm (n = 20). These morphological characteristics are consistent with the genus Colletotrichum. For molecular identification, the internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), and β-tubulin 2 (TUB2) were amplified using the corresponding primer pairs (Weir et al. 2012). All sequences were submitted to GenBank under accession numbers PX851062 (ITS), PX915664 (GAPDH), PX915662 (ACT), and PX915665 (TUB2). A phylogenetic tree based on the concatenated dataset of all gene sequences showed that the isolate clustered with C. siamense. Pathogenicity of the isolate was verified in a potted experiment at 28 ± 2°C, 70–85% relative humidity, and a 12 h light/dark photoperiod. Five sugarcane seedlings were spray-inoculated with a conidial suspension (1×10⁷ conidia/mL), while five control plants were sprayed with sterile water. Typical leaf spot symptoms developed on inoculated leaves within 10 days, while controls remained asymptomatic. Colletotrichum siamense was re-isolated from symptomatic tissues and confirmed by morphology and molecular analysis, fulfilling Koch's postulates. This species has been reported to cause leaf diseases on various hosts, including Capsicum annuum, Macropanax rosthornii, and Annona muricata (de Silva et al. 2019; Beltrán-Peña et al. 2023; Wang et al. 2024). To our knowledge, this is the first report of C. siamense causing ring spot on sugarcane in China. This pathogen poses a threat to production by potentially reducing yield, which could result in significant economic losses for the region. These findings provide a key basis for epidemiological surveillance and sustainable disease management to minimize economic losses.