Bipolaris leaf spot of rice

Bipolaris oryzae is a highly destructive fungal pathogen belonging to the kingdom Fungi and the order Pleosporales. It is the primary causal agent of brown spot disease in rice, which is recognized globally as a major limiting factor in rice production, affecting crop vigor from the seedling stage through to grain maturation.

The fungus manifests through characteristic brown, oval-shaped lesions on leaves, sheaths, and glumes. In severe infections, these spots coalesce, causing the leaf tissue to wither and die prematurely. On the grain, the pathogen causes distinct dark brown to black spots, often referred to as black mold, which significantly compromises both the quantity and the physical quality of the harvested rice.

The lifecycle of Bipolaris oryzae is driven by its ability to survive in infected seeds, soil debris, and collateral host weeds. The fungus spreads primarily through airborne conidia dispersed by wind and splashing rain. Development is favored by high humidity (85-100%) and moderate-to-high temperatures ranging from 25°C to 30°C, conditions frequently found in tropical and temperate rice-growing regions.

The economic impact is profound, as the disease can cause yield losses ranging from 5% to over 50% under epidemic conditions. Beyond yield reduction, the infection interferes with the physiological processes of the rice plant, such as photosynthetic efficiency, and increases the vulnerability of the crop to other stressors and secondary infections.

Effective management and control strategies rely on an integrated approach:

• Selection of resistant or tolerant rice cultivars.
• Treatment of seeds with effective chemical or biological fungicides.
• Adoption of balanced fertilization, particularly ensuring adequate potassium levels to strengthen cell walls.
• Sanitation practices, including the removal and destruction of crop residues where the fungus overwinters.
• Timely application of systemic fungicides during the reproductive stages of the crop.