Visual diagnostic · India · monsoon

The four monsoon diseases on Indian terraces — how to tell them apart.

Updated 8 May 2026 · 8 min read

You're here because something on your tomato (or chilli, or brinjal) doesn't look right and you can't tell what. This guide is built to be scannable — find the disease that matches what you're seeing, jump to the treatment, get back to your plant. The four diseases below cause about 90% of monsoon plant deaths on Indian terraces.

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Quick-decision matrix

One question narrows it to two diseases. Two questions usually gets you to one.

What you see	Likely disease	Jump to
Yellow patches with brown rings on lower leaves	Early blight	Read fix →
Plant limp at midday despite wet soil	Root rot	Read fix →
Curled, leathery NEW leaves at the top	Tomato leaf curl (TYLCV)	Read fix →
White powdery patches on upper leaf surface	Powdery mildew	Read fix →

Early blight

Alternaria solani — fungal

What you see: Yellow patches with concentric brown rings inside them. The ring pattern is the giveaway — almost no other Indian tomato disease has it. Patch starts coin-sized; left alone it spreads outward and joins other patches.
Where on the plant: Lower leaves first, working upward. Leaves nearest the soil go first because spores splash up from the bare soil during rain.
How fast it moves: 10–14 days from first patch to losing a leaf in monsoon humidity. Whole plant defoliation possible in 4 weeks.
Treatment: Pluck affected leaves and bin them — don't compost (spores survive). Spray neem oil (5 ml in 1 L water + a drop of dish soap) every 7 days. For severe cases on high-value plants, copper-oxychloride fungicide at the label dosage.
Prevention: 1-inch mulch layer to stop soil splash. Water at the base, not over leaves. Stake the plant so airflow reaches the lower canopy. Switch to potash-leaning feeds in monsoon.
When to call the agronomist: If three or more plants show the rings simultaneously, or if patches are still spreading after 10 days of neem treatment.

Root rot

Phytophthora / Pythium — fungal-oomycete

What you see: Plant looks limp at midday despite wet soil — that's the giveaway. Lower leaves yellow uniformly (no ring pattern). Stem at soil line may be soft or darkened. Pull the plant gently — roots are mushy, brown, smell rotten instead of crisp white-tan.
Where on the plant: Root system first; the leaves are a downstream symptom. By the time leaves yellow, the root mat is usually 50%+ damaged.
How fast it moves: Plant can collapse in 5–7 days once leaves start yellowing. The disease has been working for weeks before that.
Treatment: Hard truth — by the time you see leaf symptoms, recovery is unlikely on the affected plant. Pull it. Discard the soil. The honest fix is for the next plant, not this one.
Prevention: This is 100% a drainage problem dressed up as a disease. The 30-second drain test on every pot (see /guides/monsoon-drainage). Lift pots off the floor. Don't reuse soil from a rotted pot — sterilise or discard.
When to call the agronomist: If multiple pots fail in the same week, the cause is likely your terrace's drainage layout, not individual plants. A 5-minute video call can spot the layout problem.

Tomato leaf curl

TYLCV / ToLCV — viral, vectored by whitefly

What you see: Curled, leathery NEW leaves at the top. Leaves cup upward at the edges, get thick and leathery, often pale-green or yellow at the veins. Plant slows down, sets fewer fruit. Look under any healthy-ish leaf for tiny white insects (whitefly).
Where on the plant: New growth at the top. Existing mature leaves stay relatively normal. The contrast is what makes it spottable.
How fast it moves: Once infected, the plant carries the virus for life. New growth gets progressively worse over 4–6 weeks. Whitefly spreads to neighbouring plants in days.
Treatment: No chemical cure for the virus. Stop the spread: yellow sticky traps above the canopy, neem oil twice a week for two weeks, quarantine the infected plant. Keep watering and feeding normally so existing fruit ripens. After harvest, pull and bin (don't compost).
Prevention: Plant TYLCV-tolerant varieties — 'Arka Rakshak', 'Arka Samrat' (IIHR/ICAR releases). Yellow sticky traps as a preventive, not just curative. Remove weeds — whiteflies host on common Indian weeds like parthenium.
When to call the agronomist: If the curl looks like virus but the AI plant doctor returns 'uncertain', or if your favorite variety (Pusa Ruby, Heirloom Cherry) keeps getting hit year after year — variety advice for your specific city is worth the call.

Powdery mildew

Erysiphe / Leveillula taurica — fungal

What you see: White, powdery patches on the upper surface of leaves — looks like someone dusted flour on the plant. Patches start small + circular; they spread and join. Eventually the leaf yellows underneath the powder and falls off.
Where on the plant: Upper surface of leaves first. Mid-canopy. Less common on lower leaves (early blight territory) and uncommon on new top growth (leaf-curl territory) — its placement is usually distinctive.
How fast it moves: 7–10 days from first patch to a powdered canopy in humid conditions. Faster than most Indian growers expect because it's sometimes thought of as a 'cool-weather' disease — it hits Indian terraces in September too.
Treatment: Spray a milk solution (1 part dairy milk to 9 parts water) every 5 days for two weeks — well-evidenced on cucurbits, also works on tomato/chilli. OR neem oil at the standard 5 ml/L dose. Remove the worst-affected leaves. Water early in the morning so leaves dry before evening.
Prevention: Spacing — don't crowd pots. Sun exposure — 5+ hours of direct light. Avoid wetting leaves during evening watering. Susceptible crops on the Indian terrace: brinjal, cucumber, lauki, chilli — keep an eye on these.
When to call the agronomist: If a milk + neem regimen doesn't slow it down within 10 days, or if it's spreading from one crop family to another (cucurbit → solanaceae) — that's unusual and warrants a look.

What we'd ask you NOT to do

Don't spray a "broad-spectrum" pesticide on a virus. TYLCV is not killed by chemicals — they only kill the whitefly vector. Use the targeted approach above.
Don't use copper fungicide every week as a "preventive". Copper builds up in soil and harms the next season's plant; it's a treatment, not a routine.
Don't pull a healthy-looking plant just because a neighbour got sick — diagnose each plant on its own.
Don't compost diseased plant material. Bin it. The spores survive home composting easily and re-infect next year.

Two or more diseases at once?

That's not unusual in monsoon and it changes the priority. A 5-minute video call lets the agronomist see the whole terrace and tell you which plant to save first. Free, no payment.

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References: ICAR–IIHR variety + treatment recommendations; CABI Plantwise knowledge base; field experience from Lucknow + Noida + Indo-Gangetic plain. Treatment plans are indicative — for recurring or high-value cases get an agronomist on the line. Updated 8 May 2026.