Late blight vs early blight on tomatoes
Compare where lesions begin, how they look and which weather preceded them before deciding whether a tomato problem is early or late blight.
Answer first
Early blight commonly starts on older lower leaves with dry brown lesions that develop concentric rings. Late blight can move rapidly through leaves and stems in cool, wet, humid weather, producing irregular water-soaked or oily-looking lesions and sometimes pale growth beneath a lesion. Treat rapid spread in favourable late-blight weather as urgent, but confirm the diagnosis before choosing a control.
Use pattern, position and pace
A single brown spot is not a diagnosis. Look at several leaves, both sides of each lesion, stems and fruit. Note where the problem began and mark one lesion so you can see how quickly it changes over the next day.
Weather changes probability, not identity. Cool wet conditions strengthen the late-blight suspicion; they do not prove that every damaged leaf is infected.
- Early blight: usually older lower foliage first; dry brown spots often show target-like rings.
- Late blight: irregular, water-soaked or oily lesions; rapid canopy and stem spread can follow cool humid weather.
- Septoria: many smaller circular leaf spots with pale centres; fruit is usually not the main target.
- Non-disease damage: scorch, nutrient stress and spray injury often follow a different whole-plant pattern.
Act in the right order
Photograph the whole plant and close lesions before removing anything. Avoid working through wet foliage, isolate suspect material from healthy plants, and use clean tools. Open the detailed disease entry to compare symptoms and local favourable hours.
If late blight is plausible and spreading quickly, seek local diagnostic or extension advice. Product availability and legal labels vary by country; a weather window never overrides the product label.
Prevent the conditions you can control
Space and support tomatoes so air moves through the canopy, water the soil rather than the leaves, and irrigate early enough for accidental wetting to dry. Remove volunteer potato and tomato plants that can bridge seasons where local guidance recommends it.
Keep a crop and disease note in the planner. Rotation cannot stop spores arriving on the wind, but it helps with several soil- and debris-associated problems that are easily mistaken for blight.
Useful next actions
Sources and method
- Late Blight of Tomato — University of Connecticut IPM
- Late Blight of Tomatoes — West Virginia University Extension
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