Symptoms of the disease are commonly observed in southern rice-growing regions. The primary damage occurs on the leaf sheaths and leaves, with the leaf sheaths being the first to show signs of infection. Initially, the affected areas appear wet and black, gradually expanding into elliptical or spindle-shaped lesions measuring 2-3 cm in length, often surrounded by brown to dark brown veins. In the later stages, the leaf sheaths of the flag leaves are frequently affected, turning yellow-green. Gray and white granular calcareous sclerotia may be found within the sheaths, though soft rot is generally not present. The tip of the infected leaf turns yellow first, then spreads along the veins toward the base. The diseased sheaths often contain white mycelium and calcareous granules, with a visible white mold layer composed of pathogenic mycelium and conidia. This condition is often mistaken for discolored leaf sheaths caused by larvae infestation of *Sesamia indicum*, so careful differentiation is essential.
The pathogen responsible for the disease is *Cylindrocladium scoparium* (also known as *Sporotrichum paracasei*), a fungus belonging to the subphylum Basidiomycota. Its conidiophores are colorless, with 2-3 forked or rounded branches that bear conidiospores. The conidia are colorless, cylindrical, and septate, measuring between 49–76 × 3–5 μm. The pathogen can grow at temperatures ranging from 5°C to 35°C, with optimal growth occurring between 25°C and 35°C. Besides rice, it can also infect barley and buckwheat.
The transmission routes and environmental conditions that favor disease development are still not fully understood. Early observations suggest that the disease tends to occur during the tillering stage of rice, particularly after the peak of tillering, especially in fields with poor drainage and prolonged flooding.
The disease is most prevalent in June, when water levels in the field are high. There is a significant difference in resistance among rice varieties, with japonica rice showing greater resistance compared to other types.
To manage the disease, it is recommended to: (1) select and plant resistant rice varieties in affected areas; (2) implement proper fertilizer and water management, drain excess water promptly, and remove infected plants to prevent further spread of the pathogen. These measures can help reduce the impact of the disease and improve crop health.
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