Potato Late Blight: How to Identify, Prevent & Control It (India)

What late blight looks like

• Leaves: dark, water-soaked patches — often starting at leaf tips and edges — that quickly turn brown and papery. In humid morning air you may see a fine white fungal growth on the underside around the edge of the spot.
• Stems: dark-brown to black lesions that girdle and collapse the stem.
• Tubers: reddish-brown, granular rot spreading in from the skin — these tubers rot in store and infect the rest.
• Speed: it spreads fast in the right weather — a field can go from a few spots to widespread collapse in days.

The weather that triggers it — spray before, not after

Late blight needs cool, damp, cloudy weather — roughly 10–20 °C with high humidity and leaves that stay wet (fog, dew, drizzle, or overhead irrigation). This is "blight weather".

The single most important idea in blight control: a protectant spray works before the spore lands, not after the plant is infected. So you watch the forecast and protect the crop ahead of a cool damp spell — waiting until you see lesions is already late.

Prevention that works

• Grow a variety with blight tolerance where one is available for your zone.
• Start with certified, disease-free seed tubers — infected seed and leftover cull piles are where blight overwinters.
• Give the crop airflow: proper spacing so the canopy dries quickly; avoid overhead irrigation in the evening.
• Earth up well so spores washing off the leaves cannot reach the tubers.
• Destroy volunteers and cull piles near the field — they are a launch pad for the disease.

The spraying strategy (protectant-first)

Blight is managed with a preventive spray programme timed to the weather, not a one-off cure:

• Apply a protectant (contact) fungicide class on a schedule, and tighten the interval when blight weather is forecast.
• When pressure is high, growers rotate in systemic/curative classes — and rotate between different modes of action to avoid resistance.
• Cover the crop thoroughly, including lower leaves; the disease starts where the canopy stays wet.

This guide does not print products or doses — the right chemical, rate and pre-harvest interval depend on the label and what is cleared in your state, and misuse breeds resistance. Confirm with your local agriculture officer or KVK, or ask YieldAI with a photo of the damage.

If blight is already in the field

• Remove and destroy affected foliage where the outbreak is small and localised (do not compost it in the open).
• Stop overhead irrigation — you are watering the disease.
• If the crop is badly hit and near maturity, dehaulm early (cut and remove the tops) to stop spores reaching the tubers, then wait before digging.
• Harvest only in dry weather and never store blight-infected tubers — one rotten tuber spoils the store.

Growing potato in your state

See the full crop guide and your region page:

• Potato cultivation guide — the full season, seed, planting and storage.
• Potato in Uttar Pradesh · West Bengal · Bihar · Punjab · Gujarat