Symptoms of this rice 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 oval or spindle-shaped lesions measuring 2-3 cm in length. These lesions are often surrounded by brown to dark brown margins. In the later stages, the sheaths of the flag leaves become infected, turning yellow-green. Gray and white granular sclerotia, which are calcareous in nature, can often be found within the leaf sheaths, although soft rot is generally not present. The tip of the infected leaf turns yellow first, then the discoloration spreads along the veins toward the edges. The diseased sheath may contain white mycelium and calcareous granules, with a visible layer of white mold on the surface—this is made up of the pathogen's mycelium and conidia. This disease can sometimes be confused with the discolored leaf sheaths caused by larvae infestation of *Sesamia indicum*, so careful differentiation is necessary.
The causal agent, *Cylindrocladium scoparium* (also known as *Sporotrichum paracasei*), is a fungus belonging to the subphylum Basidiomycota. The conidiophores are colorless and have 2-3 forked or rounded branches, from which conidiospores develop. The conidia are colorless, cylindrical, and septate, with dimensions ranging from 49 to 763–5 μm. The pathogen thrives between 5°C and 35°C, with optimal growth occurring at 25°C to 35°C. Besides rice, it can also infect barley and buckwheat.
The transmission routes and environmental conditions that favor disease development are not yet fully understood. Early observations suggest that the disease tends to occur during the tillering stage, particularly before and after the peak of rice growth, especially in fields with poor drainage and prolonged flooding.
The disease is most severe in fields with high water levels, and the highest incidence typically occurs in June. There is a clear difference in resistance among rice varieties, with japonica rice showing greater resistance compared to other types.
To manage the disease, farmers should focus on selecting resistant rice varieties in affected areas. Additionally, proper fertilizer and water management is crucial. Fields should be drained promptly, and infected plants should be removed and treated to prevent further spread of the pathogen. Maintaining good field hygiene and monitoring for early symptoms can significantly reduce the impact of the disease.
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