A Rice Disease That Does Not Respond to Mancozeb
A farmer in Nalanda has 1.2 ha of paddy, 50 days after transplanting. For the last week there has been continuous drizzle and high humidity, with night temperatures around 22–24°C and day temperatures around 30°C. The crop received 60 kg N/ha in two splits and the field has been kept continuously flooded. Leaves show oval to spindle-shaped spots with grey centres and reddish-brown margins; some spots are on the neck and panicle. The farmer sprayed mancozeb 75 WP at 2.5 g/L four days ago, but new spots are still appearing. He now wants to mix carbendazim with chlorpyriphos and spray again, because 'the first medicine was weak'. A neighbour's field has similar yellowing but no grey-centred spots, and the farmer is unsure whether his field has brown spot or bacterial blight.
1. Identify the most likely disease and distinguish it from brown spot and bacterial leaf blight using the symptoms described.
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The spindle-shaped or oval lesions with grey centres and reddish-brown margins, together with neck and panicle infection, fit rice blast (Magnaporthe oryzae). Brown spot (Bipolaris oryzae) usually produces smaller, dark-brown oval spots, often on poor or nutrient-deficient soils, without the characteristic grey centre. Bacterial leaf blight starts as water-soaked streaks from the leaf tip or margin, often with a yellow halo, and causes wilting and drying rather than discrete grey-centred spots. The weather-continuous drizzle, high humidity and moderate night temperature-also favours blast, especially where nitrogen is high. The neighbour's 'yellowing without grey centres' is not enough to diagnose blast, so the two fields should not be treated identically.
2. Why did mancozeb fail? Recommend a spray with dose and timing, and state what should not be mixed.
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Mancozeb is a protectant fungicide: it forms a protective film on the leaf surface but does not move into tissue to cure established infections. It was sprayed four days after symptoms were already visible, so it could not stop lesions that had already developed; the new spots are likely from infections that occurred before or during the spray. Use a systemic or penetrating fungicide effective against blast, such as tricyclazole 75 WP at 0.6 g/L (about 600 g/ha in 600–800 L water) or isoprothiolane 40 EC at 1.5 mL/L. Spray at boot-leaf and again at 50% flowering if the weather remains humid, with 10–15 days between sprays. Carbendazim is not the best choice because benzimidazole resistance is common in blast, and chlorpyriphos is an insecticide with no role here; tank-mixing it adds cost, may harm natural enemies and increases phytotoxicity risk.
3. What cultural measures would reduce spread, especially nitrogen and water management?
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Reduce nitrogen rather than adding more. The crop has already received 60 kg N/ha by 50 days; if the leaves are dark green and succulent, skip the remaining top-dress or apply only a small dose (15–20 kg N/ha) at panicle initiation if the crop looks pale. Excessive nitrogen makes leaves soft and prolongs wetness, favouring blast. Apply potassium if it was not applied, because adequate K strengthens stems and is associated with better disease tolerance. Keep shallow water (2–3 cm) rather than deep flooding, and avoid late-evening irrigation that keeps leaves wet overnight. Do not drain the field completely during blast; drought stress can worsen the disease. Remove heavily infected panicles only if practical, and plan a resistant variety and seed treatment for the next season.