Brown spot of rice

The pathogen survives between growing seasons as dormant mycelium or conidia on infected seeds, crop residues, and collateral weed hosts. Primary inoculum is disseminated by wind, rain splashes, and irrigation water to healthy rice plants in the vicinity.

Development of the fungus is highly favored by environmental conditions such as high relative humidity (above 90%) and temperatures between 22°C and 30°C. Stressed plants, particularly those suffering from nutrient deficiencies, are significantly more susceptible.

The fungus produces toxins that damage the host tissues, facilitating rapid colonization. As the infection progresses, the pathogen forms new conidia on the necrotic lesions, leading to polycyclic disease spread throughout the rice growing season.

The pathogen significantly reduces the photosynthetic area of the leaves, causing premature senescence. Severe infections lead to poor grain filling, low milling quality, and substantial yield losses which can exceed 40% in vulnerable, susceptible rice cultivars.

The disease affects all aerial parts of the rice plant, including the coleoptile, leaves, leaf sheaths, stems, and the panicle branches. Young seedlings are often killed before emergence, while older plants suffer from leaf spotting and panicle blight.

Symptoms may appear shortly after seedling emergence. However, the most destructive phase occurs during the reproductive stages, specifically from the panicle initiation stage through the grain filling period under prolonged rainy conditions.

Typical symptoms include small, oval to circular spots on leaves that are initially dark brown or reddish-brown. As lesions enlarge, they often develop a pale yellow halo, and in humid weather, they become covered with a velvety olive-gray fungal mass.

When the infection reaches the panicles, the grains exhibit dark brown to black spots and may become completely blighted. Infected grains are often lightweight, discolored, and exhibit poor germination, serving as a primary source for future outbreaks.

Effective management requires an integrated approach starting from seed treatment to the application of fungicides during the critical developmental stages of the rice crop.

• Utilize high-quality, disease-free seed treated with systemic fungicides.
• Improve soil fertility management to ensure adequate potassium and silicon uptake.
• Remove and destroy crop stubble to reduce initial inoculum levels in the field.
• Plant resistant or tolerant rice varieties recommended for the specific region.
• Apply foliar fungicides during the tillering and heading stages if environmental conditions favor disease development.