Tomato leaf disease identification: a visual field guide
Most tomato losses start as a mark on a leaf that was misread for a week. This guide sorts the common tomato leaf problems by what the lesion looks like, where on the plant it appears first, and how fast it spreads — the three things that separate them.
Start from the bottom of the plant
Where the first symptom appears is the strongest single clue. Fungal diseases that splash up from the soil — early blight and septoria — always begin on the oldest, lowest leaves. Virus and nutrient problems usually show first at the growing tip. Late blight can start anywhere and moves fastest of all.
Telling the four look-alikes apart
Early blight makes brown spots with concentric rings, like a target, ringed by a yellow halo. Septoria makes many smaller circular spots with pale grey centres and dark edges — dozens on a single leaflet. Bacterial spot lesions look greasy and water-soaked before they dry. Late blight is the one to fear: irregular grey-green patches that go brown-black within two days, with white fuzz on the leaf underside in humid mornings.
If the lesion is dry, angular and bounded by the leaf veins, suspect bacterial speck or spot. If it crosses veins freely and expands overnight, treat it as late blight and act immediately.
- Target rings + lowest leaves = early blight.
- Many small grey-centred spots = septoria leaf spot.
- Greasy water-soaked spots = bacterial spot.
- Fast grey-green rot + white underside fuzz = late blight.
- Upward curling, thickened, stunted new leaves = leaf curl virus (whitefly-borne).
- Uniform yellowing between veins on old leaves = nitrogen or magnesium shortage, not disease.
Organic response by diagnosis
For early blight and septoria, remove and burn affected lower leaves, mulch to stop soil splash, and spray a copper or potassium-bicarbonate solution every 7-10 days. For bacterial spot, stop overhead irrigation, avoid working the rows while wet, and use copper preventively.
Late blight in humid weather is rarely stopped organically once established. Pull and destroy infected plants — do not compost them — and protect the remaining block. Leaf curl virus cannot be cured; control the whitefly vector with yellow sticky traps and neem, and remove infected plants early.
Preventive routine that removes most of the risk
Rotate away from tomato, potato, chilli and brinjal for at least two seasons. Stake plants so airflow dries the canopy by mid-morning. Water at the base early in the day. Space wider than feels comfortable — most blight outbreaks are humidity problems before they are pathogen problems.
Frequently asked
Can I eat tomatoes from a plant with leaf blight?
Fruit with no lesions from an early-blight plant is safe. Discard any fruit showing firm brown patches, which indicates the infection reached the fruit.
How quickly does late blight kill a crop?
In cool, wet weather an unprotected field can collapse in 7-10 days. Same-day action matters.
Is leaf curl caused by too much sun?
Heat causes a temporary rolling that flattens overnight. Viral leaf curl is permanent, thickens the leaf, and stunts new growth.
Not sure what you are looking at?
Photograph the leaf and PlantIQ diagnoses it on your device — no signal needed.