Rice leaf blight, also known as the disease caused by *Bipolaris oryzae*, is a widespread rice infection that occurs across all rice-growing regions in the country. The disease can affect rice plants from the seedling stage up to the harvest period. It primarily damages above-ground parts of the plant, particularly the leaves. When seeds are infected, the coleoptile becomes brown, the shoot fails to emerge, and the cotyledons wither. In the seedling stage, lesions on the leaves and sheaths typically appear as elliptical spots, similar in size to flax seeds, dark brown in color, and sometimes they expand into elongated stripes. These lesions often lead to the death of the affected tissue.
As the disease progresses in adult plants, small brown spots first appear on the leaves, gradually expanding into elliptical shapes resembling sesame seeds. The center of the lesions turns brown to gray, while the edges remain dark. A yellow halo surrounds these spots, and in severe cases, irregularly shaped lesions develop. The affected leaves dry out from the tips inward, turning brownish. Black fungal growth (conidia and spores) appears on dead seedlings. On the leaf sheaths, lesions start as oval, dark brown spots with brown edges and a water-soaked appearance, later developing into large, irregular spots with a central taupe color.
Infected panicle necks and branch stems turn dark brown, leading to premature drying of the panicles. In early grain stages, the infection spreads throughout the grain, causing a reduction in grain quality. In later stages, the lesions become smaller and less distinct. Infected grains become brittle and weak. Under humid conditions, a black fungal layer forms on the diseased tissues, consisting of conidiophores and conidia of the pathogen.
The pathogen, *Bipolaris oryzae* (formerly *Helminthosporium oryzae*), belongs to the genus *Deinophyta*. Its conidiophores grow in 2–5 bundles from stomata, unbranched, slightly curved, and septate. They range in length from 24 to 122 μm, with 3–11 septa. The fungus is an ascomycete and is only found in culture media, not in natural environments.
The disease spreads through overwintering mycelium in infected plant debris or on seeds. Conidia can survive for 2–3 years under dry conditions, while latent mycelium may persist for 3–4 years. Mycelium loses viability after one winter in the soil. Infected seeds can directly introduce the pathogen to seedlings. Wind and rain can carry conidia to new fields, where germinating hyphae can invade through stomata or directly penetrate the host. High humidity, warm temperatures, and heavy dew promote rapid disease development. Acidic soils, sandy soils, phosphorus deficiency, and potassium deficiency increase the risk of infection. Dry field conditions can also favor the disease.
The optimal temperature range for mycelial growth is 5–35°C, with the best growth occurring at 24–30°C. Conidial formation is most effective between 8–33°C, with 30°C being ideal. Spore germination requires temperatures between 2–40°C, with the best germination at 24–30°C. Germination must occur in the presence of water droplets, with relative humidity exceeding 92%. At 25–28°C and under saturated humidity, the fungus can infect the host within 4 hours.
To manage the disease, it is essential to reduce the source of infection by properly disposing of infected straw. Use disease-free seeds or treat seeds with disinfectants. Apply decomposed compost as a base fertilizer, and ensure timely top-dressing with adequate phosphorus and potassium. Potassium application improves plant resistance. For acidic soils, apply lime to adjust pH. Avoid prolonged flooding and practice shallow irrigation to improve air circulation. Chemical control measures are similar to those used for rice blast disease.
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