Brown Spot (Rice)

Brown Spot of Rice, caused by the necrotrophic fungal pathogen *Bipolaris oryzae* (teleomorph *Cochliobolus miyabeanus*, formerly *Helminthosporium oryzae*), is one of the most widely distributed foliage and grain diseases affecting paddy crops (*Oryza sativa*) across tropical and subtropical regions. Historically linked to the Great Bengal Famine of 1943, this disease primarily attacks nutrient-starved, drought-stressed, or potassium-deficient rice crops, causing severe leaf blighting, seedling mortality, and darkened, unmarketable grains.

Identifying Brown Spot involves looking for numerous small, oval, dark brown or sesame-seed-shaped spots distributed across leaf blades. Young lesions start as tiny reddish-brown specks that quickly expand into circular or oval lesions (0.5 to 1 cm long) featuring a dark brown-to-maroon border surrounded by a prominent light yellow halo. Unlike the spindle-shaped, grey-centered lesions of Rice Blast, Brown Spot lesions remain distinctly dark brown throughout. On mature grains, the fungus causes black discoloration, glume spot rot, and empty grain filling ('pecky rice').

The fungus overwinters on infected seed coats, crop residues, and wild grass hosts. Spores are dispersed rapidly by wind and splashing rain under high humidity (>88% RH) and temperatures between 25°C and 30°C. Disease severity is strongly correlated with physiological plant stress, particularly low soil potassium, silicon, or nitrogen. Organic control relies on biological seed treatment with *Pseudomonas fluorescens*, soil applications of potash and rice husk ash (bio-silica), and crop rotation. Chemical management includes foliar sprays of mancozeb, edifenphos, or iprodione. For comprehensive field diagnostic guides, read our blog post on Rice Blast & Paddy Disease Identification, or explore related crop threat profiles like Rice Blast and Bacterial Leaf Blight (Rice).