Potato Blight: Why It Happens, Early Symptoms and How to Prevent It

Blight is the most damaging disease of the potato crop. When the weather turns favourable, it can dry up the leaves and stems of a whole field within days, and it hurts both the tuber yield and how well the potatoes keep in storage. Late blight (Phytophthora infestans) is the disease that destroyed Ireland's potato crop in the 1840s and caused the Great Famine there.

The good news is that blight can be held back if you recognise the early symptoms and act at the right time. In this guide we explain in simple words why potato blight happens, what the early symptoms look like, and how to protect the crop from planting to storage.

Potato blight at a glance

Farmers use the word "blight" for two different diseases. Their symptoms and their treatment are a little different, so learn to tell them apart first.

Why does potato blight happen?

Blight does not come from one single mistake. It spreads fast when three things meet: a source of the disease, suitable weather, and a weak or crowded crop. The main reasons are:

• Infected seed tubers: Diseased or spotted tubers used as seed carry the disease into the field. This is considered the biggest source of late blight.
• Crop residue and volunteer potatoes: Rotten potatoes, cut haulms and potatoes that sprouted by themselves near the field or bunds keep the disease alive until the next season.
• Cool, humid, cloudy weather: Fog, clouds, dew and drizzle keep the leaves wet for a long time. In this, spores are carried by the wind, land on leaves and grow.
• Dense planting and heavy foliage: Air movement is poor and humidity stays high between crowded plants.
• Unbalanced nutrition: Low nitrogen weakens the plants and early blight sets in sooner. Too much nitrogen makes the plant soft and lush, which is easier for the disease. Low potash also lowers the plant's resistance.
• Evening or night irrigation and frequent light irrigation: Leaves and soil stay wet for longer.
• Growing potato, tomato or brinjal again and again in the same field: These crops belong to the same family, so repeating them lets the disease settle in the soil. Tomato gets the same blight.

When is the risk of late blight the highest?

The risk of late blight is the highest when the temperature is between 10 and 20 °C, the relative humidity is around or above 90 per cent, and the sky is cloudy, foggy or drizzling. If all three continue for two to three days, the disease grows very quickly.

How to recognise the early symptoms of blight

Catching the disease early is the best protection. The symptoms of the two diseases are listed separately below. Walk through your field in the morning, when the leaves carry dew, because the symptoms are easiest to see then.

Early symptoms of late blight

• Light green to dark brown, water-soaked-looking spots appear on the edges or tips of the leaves.
• In the morning or in humid weather, a white, cottony mould shows on the underside of the leaf, around the edge of the spot.
• The spots grow fast and turn black, the leaves shrivel and drop, and the whole plant looks scorched.
• Brown-black streaky marks form on stems and stalks, which can weaken them until they break.
• A faint rotting smell may come from the field.
• On tubers: slightly sunken, purplish-brown patches on the skin and a dry, brown rot just under the skin. In storage, other rotting organisms enter through these patches and turn the tuber to mush.

Early symptoms of early blight

• Small, round or angular dark brown spots appear first on the older, lower leaves.
• As the spots grow, clear rings appear inside them (like a target board), and the leaf turns yellow around each spot.
• The disease slowly moves up to the upper leaves, and the leaves dry up and fall early.
• On tubers: dark brown, sunken, somewhat hard (cork-like) patches can form.

Is it really blight?

If you are not sure, take a leaf sample to your nearest Krishi Vigyan Kendra (KVK) or agriculture officer. Waiting gives the disease time to spread.

How to prevent potato blight

With blight, prevention works better than cure. Think of protection in four steps: before planting, during the crop, monitoring, and spraying.

1. Before planting

• Use healthy, certified seed tubers. Never plant diseased, rotten or spotted tubers.
• Choose the variety carefully: some varieties, such as Kufri Jyoti, Kufri Girdhari and Kufri Himalini, are regarded as comparatively tolerant to late blight, while many popular varieties are susceptible. Ask your agriculture university or KVK for the right variety for your region.
• Seed tuber treatment: before planting, treat the seed tubers with a fungicide solution as recommended on the product label, and dry them in the shade before planting.
• Remove crop residue and volunteer potatoes from the field and bunds, and destroy them (bury them deep or burn them).
• Crop rotation: do not grow potato, tomato or brinjal again for 2–3 years. Grow wheat, maize or mustard instead.
• Keep the field well drained so that water does not stand.

2. During the crop

• Keep proper spacing. Avoid dense planting so that air can move between the plants.
• Feed in a balanced way. Not too little and not too much nitrogen, with potash in the right amount. Decide the fertiliser dose from a soil test.
• Earthing up: earth up the ridges well so that the tubers stay covered and spores washed down from the leaves do not reach them.
• Irrigate in the morning, not in the evening or at night. Postpone irrigation on foggy and cloudy days.

3. Monitoring: look at the field often

• From December to February, check the field every 2–3 days, and every day when it is cloudy, foggy or drizzling.
• Follow weather warnings and the advisories of the agriculture department. If temperatures of 10–20 °C, high humidity and cloud are forecast, do not wait for the disease to appear.
• If blight shows up in nearby farms, spray a protective fungicide in your field at once.

4. Fungicide sprays

The best rule for spraying is: before the disease arrives, use a protective (contact) fungicide; as soon as the disease shows, use a protective plus a systemic (combination) fungicide.

Remember: the doses above are general recommendations. Always read the latest product label and follow its dose, timing and precautions. About 200–250 litres of water per acre is normally needed; the volume changes with the size of the crop. Wet both sides of the leaves well, and do not use the same fungicide again and again: rotate between products.

While spraying

• Spray in the morning or evening, not in strong sun or strong wind. If rain is expected, spray before it.
• A sticker can be added if the label allows it, so that the spray stays on the leaves.
• If rain or irrigation washes the spray off, you may need to spray again.
• Wear gloves, a mask and full clothing. Wash hands and face with soap after spraying. Dispose of empty containers safely.
• Respect the pre-harvest interval (PHI): the number of days printed on the label between the last spray and digging.

What to do right away if blight has already started

1. Inspect the whole field and see how far the disease has spread. If only a few plants are affected at the beginning, pull them out carefully and destroy them outside the field.
2. Spray at once: use a combination fungicide (as shown in the table above).
3. Check again after 5–7 days and, if needed, repeat with a different product.
4. Stop or reduce irrigation, especially on foggy and cloudy days.
5. In severe infection: if the leaves are fully scorched and the tubers have already formed, cut the haulms (the vines) and dig the crop 10–15 days later. This stops the spores from reaching the tubers and lets the skin of the tubers harden.

Care during digging and storage

• Dig in dry weather and let the tubers dry in the shade for a short time.
• Sort out spotted, cut or rotten tubers. Do not store them with healthy tubers and never keep them as seed.
• Sort and dry the tubers before putting them in cold storage or any store.
• Do not throw rotten tubers near the field or bunds; bury them deep or destroy them.

Common mistakes in blight management

• Waiting for the disease to appear, even though the weather is already favourable.
• Spraying the same fungicide every time, which can make the disease tolerant to it.
• Using less or more than the label dose.
• Spraying only the top of the plant and not wetting the underside of the leaves.
• Irrigating in the evening and on foggy days.
• Keeping infected or spotted tubers as seed.
• Not removing volunteer potatoes and crop residue from the field.

Blight prevention checklist

1. Used certified, healthy seed tubers and treated them.
2. Chose a variety that suits the region.
3. Kept the field clean; removed old residue and volunteer potatoes.
4. Kept proper spacing, balanced nutrition and did earthing up.
5. Irrigated in the morning.
6. Checked the field every 2–3 days from December to February.
7. Sprayed a protective fungicide before the weather turned bad.
8. Sprayed a combination fungicide within 24–48 hours of seeing symptoms.
9. Rotated products and followed the label every time.
10. Dug in dry weather and set aside infected tubers.

Frequently asked questions (FAQ)

Why does potato blight happen?

Blight is caused by a fungus or a fungus-like organism. The main reasons are infected seed tubers, crop residue from the earlier crop, cool-humid-cloudy weather, dense planting, unbalanced fertiliser and evening irrigation.

What is the difference between early blight and late blight?

Early blight shows brown spots with rings on the lower leaves and spreads slowly. Late blight shows water-soaked dark spots on the leaf tips or edges with white mould underneath, and in suitable weather it can ruin a whole field very quickly.

In which weather does late blight come?

When the temperature is about 10–20 °C, the humidity is around or above 90 per cent, and the sky is cloudy, foggy or drizzling. In North India the risk is generally higher from late December to February.

What are the early symptoms of blight?

In late blight: water-soaked spots on the leaf tips or edges and white cottony mould underneath. In early blight: brown spots with rings on the lower leaves.

What is the treatment for blight in potato?

Before the disease, spray a protective fungicide such as mancozeb or chlorothalonil. When the disease shows, spray a combination fungicide such as cymoxanil + mancozeb or metalaxyl + mancozeb at once, and repeat after 7 days with a different product. Always follow the dose on the label.

When should I start spraying to prevent blight?

Generally from 40–45 days after planting, especially when the weather is cloudy, foggy and humid. Do not wait for the disease to appear. Take advice from the agriculture department or your KVK as per your area.

Fungicides at KisanShop

To pick the right fungicide for your field, see fungicides at KisanShop. A few examples by active ingredient are given below; read the label and dose of every product before use.

• Mancozeb 75% WP: Synergy M-45 Mancozeb 75% WP
• Chlorothalonil 75% WP: Synergy Clutch (Chlorothalonil 75% WP)
• Cymoxanil 8% + mancozeb 64% WP: Biostadt Duo Guard, Agriventure Cymoxzeb
• Metalaxyl 8% + mancozeb 64% WP: Paidavaar Menzyl
• Dimethomorph 50% WP: Parijat Brillar, Katyayani Dismiss
• Azoxystrobin + difenoconazole (for early blight): Silver Sine Topstar

Also read about other fungal diseases of vegetables in the rainy and cool season: protecting vegetables from fungal diseases in the rainy season (in Hindi).

Important note: This article is for general information. Before using a fungicide or any agricultural chemical, read the latest product label and follow the dose, timing and precautions. Recommendations can change with the variety, weather and local conditions, so take the final decision on the advice of your nearest Krishi Vigyan Kendra (KVK) or the state agriculture department. Kisan Call Centre: 1800-180-1551 (toll free).