What is late blight and how to control it

Late blight, caused by Phytophthora infestans, is the most destructive plant disease in recorded history — it triggered the Irish Famine of the 1840s and wiped out entire potato crops across Europe. On an Indian terrace, it may feel distant from history, but the threat is very real. During the kharif season (June to October), when Lucknow, Kanpur, Delhi, and Jaipur see high humidity, overcast skies, and temperatures between 15°C and 22°C at night, your tomato or potato plants in 20-litre grow bags are sitting in exactly the conditions this pathogen loves most. This page explains what late blight looks like, how it is different from early blight, and what you must do within 24 hours of seeing the first symptoms. Act fast — this disease can kill a plant in three to five days if you let it run.

What late blight looks like on tomato and potato

The first thing most terrace gardeners notice is a water-soaked, greasy patch on a lower leaf. It looks almost like the leaf got wet from inside. The colour is grey-green, and the edges of the patch are irregular — not the neat dark rings you see with early blight.

Within 24 to 48 hours, that patch turns dark brown to black. The tissue collapses and feels papery or leathery. If you look at the underside of the leaf in the early morning when humidity is high — or after a night of rain — you will often see a white or pale grey fuzzy growth. That is the sporulation of Phytophthora infestans, and it is actively releasing spores into the air around your plants.

Leaf symptoms:

Pale grey-green, oily-looking patches, usually starting on lower or outer leaves
No concentric rings (this is the key difference from early blight)
Rapid browning and blackening within one to two days
White sporulation on the underside in humid or wet conditions

Stem symptoms:

Brown to black lesions on stems, often starting where a leaf attaches
Stems can collapse if the lesion girdles the stem fully
Dark discolouration can spread upward toward the growing tip quickly

Fruit symptoms (tomato):

Firm, dark brown patches on green or ripening fruit
Fruit does not turn mushy at first — it stays firm but with a leather-like texture
Eventually the fruit rots and drops

Potato tuber symptoms (if you grow potatoes in grow bags):

Reddish-brown discolouration just under the skin
The rot is firm at first but secondary bacteria quickly make it mushy and foul-smelling

On a terrace with multiple pots close together, spores can travel a few metres in still, humid air. A plant in a 20-litre bag near the infected one can show symptoms within 48 hours. This is why you cannot wait and watch.

How late blight is different from early blight

This is the question we hear most often from gardeners in Lucknow and Delhi who are dealing with brown patches on tomato leaves. Early blight (Alternaria solani) and late blight (Phytophthora infestans) are both common in the kharif season, but they need different responses and different urgency levels.

Feature	Early blight	Late blight
Cause	Alternaria solani (fungus)	Phytophthora infestans (oomycete)
Lesion appearance	Dark brown spots with yellow halo, concentric rings (target pattern)	Greasy grey-green, no rings, irregular edges
Speed of spread	Moderate — days to weeks	Very fast — 3-5 days can kill plant
Underside sporulation	Absent or faint	White fluffy growth in humid conditions
Stem involvement	Rare	Common, can girdle stem
Fruit	Leathery black patches at stem end	Firm dark brown patches anywhere on fruit
Urgency	High but manageable	Emergency — act within 24 hours

The presence of concentric rings tells you it is early blight. The greasy grey-green colour with no rings, combined with rapid spread, tells you it is late blight. When in doubt, treat as late blight — the response overlaps and you lose nothing by acting fast.

For more detail on early blight, see our guide: What is early blight?

Why terrace gardens in India are especially vulnerable

Late blight thrives under specific conditions: temperatures between 10°C and 25°C, relative humidity above 75%, and leaf wetness lasting six or more hours. In northern India, these conditions arrive reliably every year between late June and September.

On a terrace or balcony, the risk is amplified for several reasons:

Crowded spacing: In the ground, plants have some separation. On a 150 sq ft terrace in Jaipur or Kanpur, you may have 10 grow bags sitting 30 cm apart. Spores produced on one plant land directly on the next.

Grow bag drainage: A properly draining grow bag should not stay waterlogged, but if cocopeat or potting mix has compacted over a season, water can sit on the surface and splash onto lower leaves, keeping them wet overnight.

No air circulation: Many terrace setups have a parapet wall on two or three sides, which restricts airflow. Humid air pools around plants after evening watering, keeping leaves wet through the night.

Recycled soil: If you reused potting mix from last year's season without sterilising it, Phytophthora oospores can survive in the mix and infect new plants immediately after planting.

During the kharif sowing window (June to July), when you transplant tomato seedlings — whether Mahyco or DeKalb hybrid varieties from Dehaat or a local nursery in Aminabad, Lucknow — the young plants go into the ground just as monsoon humidity peaks. That is when late blight pressure is highest.

How to respond when you see symptoms

The following steps are numbered by priority. Do not skip ahead. Time is the most critical variable with late blight.

Step 1: Act immediately — same day, within hours

If you see even two or three suspicious grey-green patches on a lower leaf, treat it as late blight until proven otherwise. Every hour you wait gives the pathogen time to produce millions of spores. A patch that covers 3 cm² in the morning may cover the entire leaf by evening under humid conditions.

Step 2: Remove and bag all infected plant parts

Put on gloves. Use clean scissors or pruning shears. Cut off every leaf or stem showing symptoms, plus two to three healthy-looking leaves immediately below and above the affected area. Place everything directly into a plastic bag. Seal it. Put it in the bin — not in your compost pile. Phytophthora can survive in compost unless the pile reaches sustained high temperatures, which most home compost heaps do not.

After pruning, wipe your scissors with a cloth soaked in dilute bleach (1 part household bleach in 9 parts water) or rubbing alcohol. This stops you carrying spores to the next plant.

Step 3: Spray copper fungicide immediately

Copper fungicides are the first line of defence against late blight. They work as a contact protectant — they do not cure infected tissue, but they prevent spores from germinating on healthy tissue. This is why speed matters: you need to apply copper before the disease spreads to healthy leaves.

Options available at agricultural input shops and online (Dehaat, BigHaat, Bayer CropScience distributors):

Copper hydroxide (Kocide 101, Kocide 3000): Mix 2.5 g per litre of water. Highly effective, widely available.
Copper oxychloride (Blitox 50, Fytolan): Mix 3 g per litre of water. Common at most kisan kendras; roughly ₹80-120 per 100 g pack.
Tribasic copper sulfate (Cupra, COC): Mix 2.5 g per litre of water.

For a standard 20L grow bag tomato plant, mix 500 ml of solution and spray until every leaf surface — top and bottom — is evenly coated. Pay particular attention to leaf undersides where spores form. Spray in the morning so leaves dry before evening. Repeat every 5-7 days while symptoms are active or while weather remains cool and humid.

Do not spray in rain or when rain is forecast within two hours.

Step 4: Use systemic fungicide if copper does not hold

If you spray copper and still see new lesions appearing within 48-72 hours, the disease is advancing faster than the copper can protect. Move to a systemic fungicide that is absorbed into plant tissue.

Ridomil Gold (metalaxyl + mancozeb): The most effective systemic option for late blight in India. Available from Syngenta distributors and Dehaat. Mix 2 g per litre of water. Spray the entire plant, including soil surface. Repeat after 7-10 days. Do not use more than two consecutive applications of the same active ingredient — resistance builds quickly in Phytophthora populations.

Cymoxanil + mancozeb (Curzate M8): An alternative systemic option. Mix 2 g per litre.

Note: Systemic fungicides should not be your first choice on a terrace — reserve them for when copper has not controlled the disease. Overuse leads to resistance, making them ineffective when you need them most.

Step 5: Consider removing the plant if infection is severe

If more than 30% of the plant's foliage shows late blight lesions, the honest assessment is that the plant is unlikely to recover and will become a continuous source of spores for everything else on your terrace. Removing it completely — bagging the whole plant and disposing of it — protects your other tomatoes, chillies, and potatoes.

This is a hard decision when you have been growing a plant for eight weeks. But late blight spreads so fast that saving one plant at the cost of four others is not a good trade.

After removal, do not plant anything in that grow bag for the current season without replacing or sterilising the potting mix. You can solarise the mix by spreading it on a tarpaulin in full sun for 4-6 weeks, keeping it moist, which builds heat that kills Phytophthora oospores.

Preventing late blight before it starts

Prevention is far easier than cure with late blight. These practices reduce your risk significantly during the kharif season:

Space your grow bags: Keep at least 45-60 cm between tomato and potato containers. On a crowded terrace, this may mean growing fewer plants — that is a better outcome than losing all of them.

Water at the base, never overhead: Use a long-spouted watering can or drip line to deliver water to the soil. Avoid wetting leaves, especially in the evening. If you must water in the evening due to your schedule, do it early enough for leaves to dry before nightfall — before 5 PM in most Indian cities during monsoon season.

Stake and prune: Keep plants upright and well-staked. Remove lower leaves that touch the potting mix or the rim of the grow bag. Good vertical airflow through the plant canopy reduces leaf wetness duration.

Preventive copper spray: During high-risk periods (overcast skies, three or more consecutive rainy days, temperatures consistently below 22°C at night), apply a preventive copper spray at 2 g per litre every 10 days even before symptoms appear. This is standard practice for commercial tomato growers in the hills (Himachal Pradesh, Uttarakhand) and is equally valid on an urban terrace in Lucknow.

Choose resistant varieties: Some hybrid tomato varieties have partial resistance to late blight. When buying seeds or seedlings from Dehaat or Mahyco, ask specifically about late blight resistance (LB resistance). Varieties like Arka Rakshak (IIHR, Bengaluru) have shown good field resistance. No variety is immune, but resistant ones give you more time to respond.

Do not reuse infected potting mix: Start fresh each season if your plants showed late blight the previous year. The ₹80-150 cost of new cocopeat mix is far less than losing a full season of tomatoes.

Frequently asked questions

Can I save a tomato plant that already has late blight?

Yes, if you catch it early. Remove all infected leaves and stems, spray copper fungicide immediately, and repeat every 5-7 days. If less than 25-30% of the plant is affected and you act on day one, recovery is possible. The plant may not produce as much fruit, but it can survive. If more than 30% is already infected, or if the main stem has lesions near the base, the better choice is to remove the plant to protect others nearby.

Is late blight the same as early blight?

No. They look similar at a glance — both cause dark lesions on tomato leaves — but they are caused by different organisms and behave very differently. Early blight (Alternaria) makes neat concentric rings and spreads slowly over weeks. Late blight (Phytophthora) makes greasy grey-green patches with no rings and can kill a plant in three to five days. Late blight is far more urgent. If you see white fuzz on the underside of leaves, that is late blight.

Is copper fungicide safe to use on vegetables I will eat?

Copper fungicides are approved for vegetable use in India and are used widely in commercial farming. Follow the label's pre-harvest interval — most copper products require 3-7 days between last spray and harvest. Rinse your harvest thoroughly. On a terrace where you are growing food for your family, use copper at the recommended rate (2.5-3 g per litre) and do not exceed the label dose. Systemic fungicides like Ridomil have a longer pre-harvest interval (typically 14 days) — check the label.

What is the white fuzz appearing on my tomato leaves at night?

That is the sporulation of Phytophthora infestans — the pathogen that causes late blight. It appears on the underside of leaves when humidity is high, usually overnight or after rain. Each white patch is producing millions of spores. This is a confirmed late blight infection. Begin treatment the same morning. Do not wait for more symptoms to develop.

My grow bag tomato got late blight. Can I reuse the soil next season?

Not without treating it first. Phytophthora produces structures called oospores that can survive in soil for months. To make the mix safe, spread it in a thin layer (5-8 cm) on a tarpaulin in full direct sun, keep it slightly moist, and leave it for 4-6 weeks during peak summer (April-June). This solar solarisation builds temperatures above 50°C which kills oospores. Alternatively, replace the mix entirely — a 50-litre bag of fresh cocopeat + perlite + compost mix costs roughly ₹250-350 and eliminates the risk.

Should I use neem oil against late blight?

Neem oil has antifungal properties and works well against several mild fungal diseases. However, late blight is caused by an oomycete, not a true fungus, and neem oil has very limited effectiveness against it. If you only have neem oil available and have no copper, apply it as a temporary measure while you arrange copper fungicide — but do not rely on neem as your primary treatment for late blight. Get copper fungicide from your nearest kisan kendra, Dehaat outlet, or order from BigHaat. The window to save the plant is short.

Got a plant problem? Use the free Plant Doctor →

Need expert advice? Book a certified agronomist →