Potato Late Blight: How to Spot, Stop, and Prevent It (Tomatoes Too)

Few crop diseases move as fast or hit as hard as potato late blight. This is the disease behind the Irish Potato Famine of the 1840s, and it has lost none of its punch. Under cool, wet conditions a healthy-looking field or tomato patch can collapse into blackened, rotting foliage within a week.

The good news: late blight is identifiable, and if you catch it early you can usually save the crop. This guide walks through how to recognise it, what causes it, what to spray, and how to keep it from coming back next season.

What is late blight, and why is it so dangerous?

Late blight is caused by Phytophthora infestans, a water mould (oomycete) rather than a true fungus, though it behaves much like one. It attacks potatoes and tomatoes most severely, but can also affect related plants such as brinjal (eggplant) and petunia.

What makes it so dangerous is speed. A single infected leaf can produce hundreds of thousands of spores, which travel on wind and splashing rain to neighbouring plants. In the right weather, the disease can spread across an entire field in three to five days and rot tubers in storage. This is not a problem you can afford to watch and wait on.

How to identify potato late blight

Late blight has a distinctive look once you know what to watch for. Inspect plants in the early morning when humidity is highest, as the key signs are clearest then.

• Water-soaked, dark lesions: Look for irregular pale-green to brown-black patches on leaves, often starting at the leaf tips or edges. They appear greasy or water-soaked rather than dry.
• White fungal growth on the underside: On humid mornings, flip the leaf over. A fine white-to-grey fuzzy ring of spores often forms at the edge of the lesion, where healthy and infected tissue meet. This is the clearest diagnostic sign.
• Rapid spread: Spots enlarge quickly and run together, and whole leaves and stems turn brown and collapse within days. Infected stems show dark brown to black streaks.
• Tuber and fruit rot: In potatoes, infected tubers develop reddish-brown, granular dry rot under the skin. In tomatoes, firm brown greasy patches appear on green and ripening fruit.

If you are unsure, you can confirm the diagnosis in seconds by scanning an affected leaf with a free app like Agrosphere, which compares the symptoms against known crop diseases and suggests treatment. With a disease this fast-moving, a quick confirmation beats a wrong guess.

Don't confuse it with early blight

Early blight (Alternaria solani) is the disease most often mistaken for late blight, but the two need different urgency and timing. Early blight produces dry, brown spots with distinct concentric rings (a target or bullseye pattern), usually on older lower leaves, and spreads slowly in warm weather.

Late blight, by contrast, looks greasy and water-soaked, carries that white mould on the underside, and explodes across the plant in cool, wet conditions. When in doubt, treat fast-spreading water-soaked lesions as late blight, since the cost of delay is far higher.

What causes it: cool, wet weather

Phytophthora infestans thrives in a specific window of conditions, and weather is the single biggest driver of an outbreak. The disease takes off when:

• Temperatures sit roughly between 10 and 24 degrees Celsius (cool, not hot).
• Humidity is high, above about 90 percent, with leaves staying wet for several hours.
• There is fog, dew, drizzle, or overhead irrigation keeping the canopy damp.

Periods of cool, cloudy, rainy weather, often called "blight weather," are the warning sign to scout your crop daily. The pathogen survives between seasons in infected tubers left in the ground or in cull piles, and in volunteer potato plants, so these are common starting points for an outbreak. Agrosphere's weather and spray-window features can help you spot high-risk days before symptoms appear.

Urgent treatment: act within days, not weeks

Once late blight is confirmed, move quickly. The goal is to stop spore production and protect healthy tissue.

1. Remove and destroy infected plants

Pull out badly infected plants immediately. Do not compost them, as the spores survive and spread. Bag and remove them, bury them deep, or burn them where local rules permit. Removing the source of spores is often the most important single step.

2. Apply an effective fungicide

For active infections, a systemic or translaminar fungicide combined with a protectant gives the best knock-down. Common, widely available options include:

• Cymoxanil + mancozeb — a curative-plus-protectant combination, typically applied at the label rate (often around 2 to 3 grams per litre of water) and repeated as the label directs.
• Metalaxyl (or metalaxyl-M / mefenoxam) + mancozeb — strong systemic action against the pathogen; alternate it with non-systemic products to slow resistance.
• Protectant contact fungicides such as mancozeb, chlorothalonil, or copper-based products to shield healthy foliage between systemic sprays.

Always read and follow the product label for the exact dose, spray interval, pre-harvest interval, and safety equipment, and check that the product is registered for late blight on your crop in your country or state. Rotate between different active ingredients rather than relying on one, because P. infestans develops resistance to single-mode products quickly. When uncertain, consult your local agricultural extension officer or krishi vigyan kendra.

3. Spray thoroughly and on time

Cover both leaf surfaces, since spores form on the underside. In high-risk weather, protectant sprays may be needed every 5 to 7 days; tighten the interval during prolonged wet spells. Spray early in the day so foliage dries quickly.

How to prevent late blight next season

Prevention is far cheaper and more reliable than fighting an active outbreak. Build these habits into your routine:

• Use certified disease-free seed potatoes and resistant or tolerant varieties where available.
• Destroy volunteer plants and cull piles, which harbour the pathogen between crops.
• Improve airflow and reduce leaf wetness: space plants well, avoid overhead irrigation, and water at the base early in the day.
• Earth up (hill) potatoes well so spores washing down are less likely to reach the tubers.
• Rotate crops: avoid planting potatoes or tomatoes in the same spot for at least two to three years.
• Scout daily in blight weather and apply protectant fungicides preventively when a wet, cool spell is forecast.
• Harvest in dry conditions and let tubers cure before storage; cool, dry storage limits rot.

The bottom line

Potato late blight earned its fearsome reputation for good reason, but it is manageable with vigilance. Learn the signs, watch the weather, and respond within days of spotting water-soaked lesions or white mould. Remove infected plants, spray the right fungicide combination at label rates, and lean on prevention so next season starts clean.

If you grow potatoes or tomatoes, make leaf-checking a daily habit during cool, wet weather. Catching late blight in its first day or two is the difference between losing a few plants and losing the whole crop.

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