Early Blight vs Late Blight in Tomato: How to Tell Them Apart (And Why It Saves Your Season)

Tomato cultivation in India covers over 9 lakh hectares, with Andhra Pradesh, Karnataka, Maharashtra, and Madhya Pradesh as the dominant producing states. In every one of these geographies, two diseases account for the majority of fungal crop losses: early blight and late blight.

They sound similar. They look similar in early stages. And they are treated very differently. Getting the identification wrong means applying a treatment that either doesn't work — or works far less effectively than the alternative.

Early Blight — Alternaria solani

Early blight is caused by the fungus Alternaria solani. Despite its name, it typically appears mid-season when plants are beginning to mature and physiological stress is higher.

Visual identification

• Starts on older, lower leaves — never on the growing tip first
• Lesions are dark brown, circular to irregular, 0.5–1.5cm diameter
• Classic concentric ring pattern inside the lesion — often described as a 'target board' or 'bull's eye'
• Yellow chlorotic halo around the lesion is common
• Lesions coalesce in severe infections, causing leaf yellowing and drop
• Stem lesions possible — dark, sunken cankers near soil level

Conditions that favour early blight

Early blight thrives in warm temperatures (24–29°C), moderate humidity, and conditions of plant stress — overcrowding, poor nutrition, drought stress. It is most aggressive in the mid-season period when lower leaves are older and nutrient-stressed.

Late Blight — Phytophthora infestans

Late blight is caused by Phytophthora infestans — technically an oomycete (water mould), not a true fungus, which has significant treatment implications. It is among the most aggressive plant pathogens known, historically responsible for the Irish Potato Famine.

Visual identification

• Can start anywhere on the plant — young leaves, older leaves, stems, and fruit
• Lesions are large, irregular, pale green to brown, with a water-soaked, 'greasy' appearance
• No concentric ring pattern — unlike early blight
• White or grey sporulation (fuzzy mould) visible on the underside of leaves in humid conditions — the clearest differentiator
• Spreads extremely rapidly — a plant can go from first lesion to complete collapse in 7–10 days in cool, wet weather
• Fruit infection: brown, firm lesions extending into the flesh

Conditions that favour late blight

Late blight is most aggressive in cool temperatures (15–20°C), high humidity, and wet conditions. In India, this makes the Kharif season in high-altitude areas and the post-monsoon period in humid coastal zones the highest-risk windows.

Why the Treatment Difference Matters

This is the critical point. Early blight responds well to contact fungicides — protectant products that coat the plant surface and prevent spore germination. These are typically lower cost and effective when applied at the right time.

Late blight requires systemic fungicides — products that are absorbed into the plant tissue and can act on established infections. Because Phytophthora is not a true fungus, some standard fungicides have little to no effect on it. Applying a contact fungicide to an active late blight infection buys days at best.

The disease continues to spread during those lost days. In late blight's aggressive progression timeline, those days represent significant additional yield damage.

When You're Not Sure

If identification is uncertain — which is common in early-stage infections before the full symptom picture develops — the conservative approach is to diagnose before treating. Applying the wrong product wastes time and money that are better spent confirming the pathogen and responding correctly.

In ambiguous cases, collect samples from multiple plants, photograph both leaf surfaces, note the weather conditions of the past 5–7 days, and seek a differential diagnosis. The few hours spent on identification are almost always worth it.