Every tomato season in India, the same story plays out on thousands of farms. A farmer spots something wrong on the leaves — dark patches, yellowing, a strange texture. He asks a neighbour. The neighbour suggests a chemical. He buys it, sprays it, and waits. Three days later, the disease has spread to half the field.
By the time the nearest agronomist arrives — typically 3 to 5 days after the first call — the season is already compromised. According to data from ICAR and UN FAO, 35% of India's crops are lost every year to something preventable. Tomato blight is one of the leading contributors to that number.
The problem is almost never the disease itself. The problem is the 72-hour window between when symptoms first appear and when a correct diagnosis reaches the farmer. In that window, a fungal infection can spread from a few plants to an entire field.
This guide gives you what you need to close that window yourself — how to identify tomato blight in its earliest stages, how to tell the two main types apart, and exactly what to do the moment you see it.
What Is Tomato Blight — and Why Does It Matter in India
Tomato blight is not a single disease. It is a category of fungal and oomycete infections that attack the leaves, stems, and fruit of tomato plants. In India, two types cause the most damage:
Early blight, caused by the fungus Alternaria solani, and late blight, caused by the water mould Phytophthora infestans — the same pathogen responsible for the Irish Potato Famine of the 1840s.
They look similar at first glance. They are treated completely differently. Applying early blight fungicides to a late blight infection — or vice versa — wastes money, wastes time, and lets the disease continue spreading unchecked.
Correct identification is not a formality. It is the difference between saving a crop and losing it.
In Karnataka, Maharashtra, Andhra Pradesh, and Tamil Nadu — which together account for the majority of India's tomato production — blight pressure is highest during the kharif season and in conditions of high humidity above 80% and temperatures between 20°C and 30°C. If you are farming tomatoes in South India during the monsoon season, you are farming in blight conditions.
Early Blight — How to Identify It
Early blight typically appears first on the older, lower leaves of the plant. It moves upward as the infection progresses.
What to look for on leaves:
- Dark brown to black spots, usually 1–2 cm in diameter
- Spots have a distinctive concentric ring pattern — rings within rings, like a target or bullseye
- Yellow halo surrounding the spot
- As infection advances, the entire leaf yellows and drops
What to look for on stems:
- Dark, sunken lesions, often at the base of the stem near the soil
- Lesions may have the same concentric ring pattern as leaf spots
- Severe stem infection can cause "collar rot" — the plant wilts suddenly from the base
What to look for on fruit:
- Dark, sunken, leathery spots, usually at the stem end
- Concentric rings visible on the fruit surface
- Black, velvety fungal growth in the centre of the lesion
Conditions that trigger early blight in India: Early blight thrives in warm, humid conditions — temperatures between 24°C and 29°C with relative humidity above 90%. It spreads rapidly during the monsoon season and in fields with poor air circulation. Plants under nutritional stress — particularly nitrogen deficiency — are significantly more susceptible.
Late Blight — How to Identify It
Late blight moves faster than early blight and is more destructive under the right conditions. An entire field can be destroyed within a week of first symptoms appearing if conditions are favourable and treatment is delayed.
What to look for on leaves:
- Irregular, water-soaked patches — not circular, not ringed, but spreading unevenly
- Patches turn brown rapidly, often with a pale green or yellow border
- In high humidity, a white, fuzzy growth appears on the underside of the leaf — this is the sporulating mycelium and is the clearest diagnostic sign of late blight
- No concentric ring pattern — this is the key visual difference from early blight
What to look for on stems:
- Dark brown to black lesions, firm and expanding rapidly
- Unlike early blight, late blight lesions tend to be larger and less defined at the edges
What to look for on fruit:
- Firm, brown, greasy-looking patches on green or ripening fruit
- The texture feels hard and leathery
- White sporulation may appear on the surface in humid conditions
Conditions that trigger late blight in India: Late blight is an oomycete — a water mould — and it requires free moisture to spread. It thrives at cooler temperatures (15°C to 25°C) combined with high humidity. It is most destructive during heavy monsoon periods, in hill districts, and in irrigated fields during cooler months. The Nilgiris, Kodagu, and parts of Himachal Pradesh are historically high-pressure zones.
Side-by-Side Identification Guide
| Feature | Early Blight | Late Blight |
|---|---|---|
| Pathogen | Alternaria solani (fungus) | Phytophthora infestans (oomycete) |
| Spot shape | Circular, defined edges | Irregular, water-soaked |
| Ring pattern | Yes — concentric rings | No |
| Colour | Dark brown to black | Brown with pale/yellow border |
| Underleaf growth | None | White fuzzy growth in humidity |
| First appears on | Older, lower leaves | Any part of plant |
| Spreads fastest in | Warm + humid (24–29°C) | Cool + wet (15–25°C) |
| Speed of spread | Moderate | Very rapid |
| Fruit symptoms | Sunken, ringed, leathery | Firm, greasy, brown patches |
Treatment Protocol for Early Blight — Indian Conditions
Once correctly identified, early blight is manageable with the right fungicide applied at the right time. The key word is right — the wrong chemical at any dose is wasted effort.
First action (immediate): Remove and destroy all heavily infected leaves. Do not compost them — burn or bury them away from the field. Reducing inoculum load is the fastest way to slow spread.
Fungicide protocol:
- Mancozeb 75% WP — 2.5 g per litre of water. Spray every 7–10 days preventively during high-risk periods. Effective contact fungicide, widely available across Karnataka agri-input dealers.
- Chlorothalonil 75% WP — 2 g per litre. Alternate with Mancozeb to prevent resistance development.
- Azoxystrobin 23% SC — 1 ml per litre. Systemic fungicide for curative action once infection is established. More expensive but effective when contact fungicides alone are insufficient.
Spray timing: Early morning or late evening. Avoid spraying in full sun — it reduces efficacy and increases phytotoxicity risk. Ensure complete coverage of both upper and lower leaf surfaces.
Pre-harvest interval: Maintain a minimum 7-day gap between last spray and harvest for Mancozeb; 14 days for Azoxystrobin. Check the product label.
Treatment Protocol for Late Blight — Indian Conditions
Late blight requires a different class of fungicides entirely. Using Mancozeb alone against late blight is inadequate — it provides only partial protection.
First action (immediate): Remove and destroy all infected plant material immediately. Late blight spreads through airborne spores and water splash — every infected leaf left in the field is a source of new infection.
Fungicide protocol:
- Metalaxyl + Mancozeb (Ridomil Gold or equivalent) — 2.5 g per litre. The most effective combination for late blight in India. Metalaxyl is systemic and works from inside the plant; Mancozeb provides contact protection.
- Cymoxanil + Mancozeb — 3 g per litre. Good curative option when infection is already established.
- Dimethomorph 50% WP — 1 g per litre. Effective systemic option, particularly useful for resistance management.
Rotation is mandatory: Do not use Metalaxyl more than twice in succession. Late blight resistance to Metalaxyl is already documented in several Indian growing regions. Alternate with Cymoxanil or Dimethomorph.
Spray frequency: Every 5–7 days during active outbreak conditions. Late blight moves too fast for a 10-day spray interval.
What to Do When You're Not Sure Which One It Is
This is the most common situation — and the most dangerous, because guessing wrong costs time and money.
When you cannot tell which blight you're looking at, do this:
- Check the underside of the affected leaf. If there is white, fuzzy growth — that is late blight. No other tomato disease produces this symptom in humid Indian conditions.
- Check for concentric rings. Clear, defined rings on a circular spot point to early blight.
- Check the weather history. Has it been cool and very wet? Late blight. Has it been warm and humid with some dry periods? Early blight.
- When in doubt, use a combination product. Cymoxanil + Mancozeb covers both diseases adequately until you have a confirmed identification.
The smarter approach is to get a confirmed diagnosis before treating. This is where AI-powered crop diagnosis has become genuinely useful in Indian field conditions.
ARCORA, built by Truffaire and deployed with FPOs across Karnataka, diagnoses tomato blight — and distinguishes between early and late blight — from a single photograph in under 2 minutes. The report includes the confirmed disease name, confidence score, severity rating, and an exact treatment protocol with chemical names, dosage, spray schedule, and pre-harvest interval, specific to the crop's growth stage and location. For FPO coordinators managing hundreds of farmers across multiple villages, it means one coordinator can deliver agronomist-level guidance at scale — at ₹33 per diagnosis versus ₹500–2,000 for a field visit.
If you're standing in a field and unsure what you're looking at, a 2-minute diagnosis is faster and more reliable than a neighbour's guess. arcora.truffaire.in
Prevention — What to Do Before Symptoms Appear
The best blight management is the kind that starts before you see a single lesion.
Variety selection: Several blight-tolerant tomato varieties are available for Indian conditions — Arka Rakshak, Arka Samrat, and several private hybrids. Tolerant varieties will not eliminate blight risk but will significantly reduce pressure during high-risk periods.
Crop rotation: Do not plant tomatoes in the same field for more than two consecutive seasons. Alternaria spores persist in soil and crop debris. Rotate with non-solanaceous crops — legumes, cereals, leafy vegetables.
Field hygiene: Remove and destroy all crop debris at the end of the season. Late blight oospores can survive in soil through the off-season.
Spacing and airflow: Dense planting increases humidity within the canopy. Maintain recommended spacing (60 cm × 45 cm for most varieties) and stake plants properly to improve air circulation.
Drip irrigation over overhead irrigation: Overhead watering wets the foliage and creates exactly the humid leaf-surface conditions that blight needs to spread. Where possible, use drip irrigation and avoid wetting leaves.
Preventive spray schedule: During high-risk periods — kharif season, heavy monsoon, temperatures in the 20–28°C range — begin a preventive Mancozeb spray schedule even before symptoms appear. The cost of preventive spraying is far lower than the cost of a lost crop.
The 72-Hour Rule
The single most important thing to internalise about tomato blight is this: the first 72 hours after symptoms appear determine the outcome of the infection.
In those 72 hours, a correctly identified and correctly treated blight outbreak can be contained. After 72 hours of delay — guessing, waiting for the agronomist, applying the wrong chemical — the disease has spread into the canopy, infected neighbouring plants, and begun releasing spores that will reinfect after the next rain.
Identify early. Identify correctly. Treat immediately.
Everything else follows from those three steps.