First Report of Bipolaris bicolor Causing Red Spot on Sugarcane in Guangdong Province of China

Sugarcane (Saccharum officinarum L.) is a perennial cash and energy crop that is predominantly cultivated in tropical and subtropical regions. In April 2025, red spot symptoms were observed on the sugarcane cultivar GT55 at South China Agricultural University (23°09′N, 113°22′E) in Guangdong Province. A survey of approximately 800 plants indicated a province-wide disease incidence of 6% to 12%. Initial symptoms on young leaves are characterized by small red spots that expand into reddish-brown streaks. In severe infections, this progression leads to extensive wilting and necrosis of the leaves. For pathogen isolation, nine samples were collected from symptomatic plants, three of which were selected for fungal isolation. Symptomatic leaf segments (approximately 0.5 cm) were then excised from the margins of the lesions. The segments were surface-sterilized by immersion in 75% ethanol for 30 s, followed by treatment with 0.1% HgCl₂ for 3 min. After three rinses in sterile distilled water, the segments were blot-dried on sterile filter paper, aseptically transferred onto potato dextrose agar (PDA) plates, and incubated at 25°C in complete darkness. After five days of incubation, nine fungal strains were isolated from three leaf samples using monosporic isolation. Six of these strains displayed similar morphological characteristics and were subsequently selected for in vitro pathogenicity testing on leaves. Wounded and detached sugarcane leaves were inoculated with six fungal strains and incubated for six days at 28°C and 80% relative humidity. Among them, strain PL2428 was identified as the most pathogenic. Its colony was light gray on the obverse and yellowish on the reverse, characterized by the formation of concentric whorls. Conidia were straight to slightly curved, smooth, terete, pale brown with a bluntly rounded tip, possessing five to nine pseudosepta and measuring 18.4 to 59 × 5.1 to 16.5 μm (n = 50) (Fig. 1). Molecular identification was performed by partial sequences of ITS (ITS4 and ITS5 primers) (White et al. 1990), TEF1-α gene (EF1-983 and EF1-2218 primers) and GAPDH gene (GPD1 and GPD2 primers) (Manamgoda et al. 2012). The ITS (PX048746), TEF1-α (PX088172) and GAPDH (PX088160) sequences were deposited in GenBank. In the BLAST analysis, the ITS, TEF1-α and GAPDH sequences all showed >99% identity with GenBank sequences of Bipolaris bicolor strains YB450101 and CBS 690.96 (Fig. 2). Thus, the fungal isolate was identified as B. bicolor. For pathogenicity testing, a conidial suspension (10⁶ conidia/mL) was applied to needle-wounded healthy sugarcane leaves (n = 6), while control plants (n = 6) were mock-inoculated with sterile water. All plants were incubated at 28 °C and 90% relative humidity under a 12-h photoperiod. At 14 days post-inoculation, inoculated leaves exhibited leaf spot symptoms identical to original field observations, whereas control leaves remained asymptomatic. B. bicolor was consistently re-isolated from the symptomatic tissue, thereby fulfilling Koch’s postulates. The experiment was repeated twice with consistent results. B. bicolor has been reported to infect a broad host range of economically important plants, including rubber tree (Liang et al., 2019) and tea (Zhong et al., 2024). This is the first report on B. bicolor causing red spot on sugarcane in China. This study establishes a pathogenic basis for the precise prevention and control of sugarcane red spot.