Brown spot in paddy

Brown spot of rice is a foliar and glume disease caused by the fungus Bipolaris oryzae (teleomorph Cochliobolus miyabeanus). It is historically referred to as the 'poor farmer's rice disease' because its severity is closely tied to nutrient-deficient and poorly managed soils.

Identification and symptoms

Lesions on leaves are small, oval to circular, sesame-seed shaped, brown with a yellow halo when young and grey-centred when older. Multiple lesions coalesce on susceptible plants and may extend onto glumes, discolouring grains. The disease is often most visible on rainfed upland fields and on nursery beds raised on starved soils.

Host crops and life cycle

The fungus survives on infected seed, stubble and grassy weed hosts. Infection is favoured by warm, humid weather, prolonged leaf wetness and especially by soils low in nitrogen, potassium, silicon and zinc. Weak, unflooded fields and stressed seedlings are most susceptible.

Damage and economic impact

Beyond foliar blighting, glume infection reduces grain quality and weight. The disease was historically associated with severe famines, including the 1942 Bengal rice famine, where it caused widespread yield collapse on already malnourished crops.

Management

Soil and nutrition: correcting nitrogen, potassium and especially zinc deficiencies is integral to control (Micronutrient Deficiency Paddy); silicon-rich soils tolerate the pathogen better.
Clean seed: certified disease-free seed and seed treatment with mancozeb or thiram at 2 g/kg.
Foliar fungicides: mancozeb plus carbendazim is a standard combination; tebuconazole, tricyclazole and propiconazole have also shown efficacy in field and in-vitro evaluations.
Cultural: maintain adequate water at sensitive stages, raise nurseries on fertile soil, and remove infected stubble.

References

Brown Spot (Helminthosporium oryzae). TNAU Agritech Portal.
Sunani S. K. et al. Brown Spot of Rice: worldwide impact and management. Plant Pathology (Wiley).