Late blight in a 46-day potato crop: what to spray when mancozeb has already failed
Ganga Prasad grows 0.5 ha of potato (Kufri Bahar) at Farrukhabad, planted on 12 November. On 28 December, at 46 days, he shows you dark grey-green water-soaked patches on lower leaves near the irrigation channel, three to five lesions per plant on about 5% of plants, a white fuzzy growth on the underside of lesions in the morning, and a few stems with blackened patches. The past four days have been 20–22 °C maximum, 10–12 °C minimum, RH above 90%, with drizzle, and two more cloudy humid days are forecast. He has already sprayed mancozeb 75 WP at 2.5 kg/ha on 12 and 22 December. The previous crop in the same field was also potato and he used seed from his own store. He wants to add metalaxyl + mancozeb "for a stronger effect" and harvest in 25 days to catch the early market. His sprayer still has last season's nozzle.
1. Confirm the diagnosis, distinguish it from the main look-alike, and explain why the two mancozeb sprays did not stop the epidemic.
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Diagnosis: late blight, Phytophthora infestans. The evidence is a combination, not any single symptom: irregular water-soaked grey-green lesions beginning on lower leaves, white fuzzy sporulation on the underside in the morning, blackened stem lesions, and weather of 18–22 °C with RH above 90% and drizzle. Spread under these conditions is explosive.
Not early blight (Alternaria solani). Early blight shows brown lesions with concentric rings like a target board, usually on older leaves in warm dry weather, with a yellow halo and no white downy growth underneath.
Why mancozeb did not hold.
- Mancozeb is a protectant: it must be on the leaf before the spore lands. It has no curative action, so any infection already inside the tissue on 12 December was unaffected.
- A 10-day interval is too long during a drizzle period when RH stays above 90%; the pathogen completes a cycle in days, not weeks.
- Rain washes the deposit off, and new leaves produced between sprays are unprotected.
- Coverage: an old nozzle gives a coarse, uneven pattern with run-off and poor coverage of leaf undersides and the lower canopy, where the epidemic started.
- Own seed plus potato after potato raises the starting inoculum, which is why the epidemic began early and near the irrigation channel.
Caution on his plan: if phenylamide (metalaxyl/metalaxyl-M) products have been used repeatedly on this farm, resistance is a real possibility; do not treat it as a guaranteed rescue.
2. Design the spray programme for the next three weeks, justifying product choice, intervals, coverage and resistance management.
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Act within 24–48 hours, because infection is already established and the weather is still favourable to the pathogen.
Product choice. He needs curative or kickback activity now, alternated with a protectant:
- cymoxanil 8% + mancozeb 64% WP, or dimethomorph 50 WP, or fluopicolide + propamocarb, at label dose; keep mancozeb alone as the protectant in the rotation.
- If a metalaxyl + mancozeb combination is used, use it once only and only where phenylamide resistance is not suspected.
Resistance management. Systemic partners should not be applied consecutively. Alternate modes of action (cymoxanil, dimethomorph, fluopicolide + propamocarb) and never exceed the label's number of applications per season per group. Two consecutive sprays of the same systemic is how field resistance is built.
Interval. Every 7 days while the weather stays humid, and re-spray after rain or overhead irrigation once the canopy dries. In this forecast, assume a 7-day rhythm, not 10.
Coverage. Use 500–700 L/ha with a hollow-cone or twin flat-fan nozzle, replace the old nozzle, add a sticker, and spray in the afternoon/evening so the deposit dries slowly and the underside is wetted. Direct the spray to the lower canopy, not just the top.
Field practice. Stop overhead irrigation. Avoid walking through the crop while foliage is wet, because that spreads sporangia. Mark hotspots near the channel and spray them first, with the nozzle directed into the canopy.
3. Advise on the 25-day harvest plan, dehaulming, pre-harvest interval and seed-tuber decisions.
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Does 25 days fit? Yes, but it allows only three to four weekly sprays, so the last spray must be planned backwards from the harvest date so that the label PHI falls inside the 25 days. Check the label for the products used (commonly around 7–10 days for cymoxanil + mancozeb and about 7 days for mancozeb; the label governs, not memory).
Dehaulming is the key step. Cut and remove the haulm 10–14 days before harvest, while the foliage is dry. This is the most effective single measure to stop spores moving down the stem onto tubers; leaving dead haulm in the field defeats the purpose.
Harvest and storage. Lift in dry weather, cure the tubers in shade for 10–15 days, and sort out any with rot before storage. Do not store rotted tubers with healthy ones.
Seed decision. Do not keep seed from this field for next season: infected tubers rot in store and carry inoculum. Buy certified seed from a reliable source. Also plan a rotation away from potato for at least two seasons where possible, destroy cull piles, and consider a variety with better late blight resistance (for example Kufri Himalini, subject to local recommendation) instead of the highly susceptible Kufri Bahar.
Records. Note weather, dates, products, doses, PHI and dehaulming date - these are what a residue or quality audit will ask for.