Brown Spots on Tomato Leaves
Brown spots on tomato leaves rarely have just one explanation. Blight — early or late — is the cause most people search for by name, but septoria leaf spot, sun scorch, and even a nutrient deficiency can look similar enough from a distance to be mixed up. The pattern of the spots, not just their color, is what actually narrows things down.
How to tell the common causes apart
Early blight starts on the oldest, lowest leaves — brown to black spots with concentric rings like a target, often with yellowing around the spot. Badly affected lower leaves drop early.
Septoria leaf spot produces many small, round spots with a dark border and a gray-white center. Tiny black specks in the middle of the spot are a telltale sign. It usually starts on the lower leaves and climbs upward in rainy weather.
Late blight is the fastest: large, irregular, water-soaked brown patches, often on the upper leaves, plus a whitish coating underneath in humid weather. In cool, wet conditions it can overrun a planting within days.
Not every brown spot is a disease. Sun scorch after a sudden change in light and potassium deficiency also cause brown discoloration, without the signs of a fungal problem.
How it spreads
Early blight and septoria survive in plant debris and soil, and reach the lower leaves in splashes from rain or watering. For all of these diseases, leaves that stay wet for long periods are what speed things up.
What helps, step by step
- Water at the base and in the morning so leaves stay dry — drip irrigation or a soaker hose works best.
- Mulch under the plants so soil does not splash onto the lower leaves.
- Stake plants and leave space between them so foliage dries quickly.
- Remove badly affected lower leaves and bin them rather than composting.
- At the end of the season, clear plant debris and avoid planting tomatoes in the same spot for a few years.
- If you are considering a fungicide, confirm the cause first and follow the product label or local extension advice. Late blight can destroy a planting within days in cool, wet weather, so do not wait if you suspect it.
Possible Causes
- Late blight (Phytophthora infestans) — dark, water-soaked spots, often with a pale halo, plus a whitish coating on the underside of the leaf in humid weather; spreads noticeably within days
- Early blight (Alternaria solani) — concentric rings inside the spot, like a target
- Septoria leaf spot — smaller, round spots with a dark border and a lighter center
- Sun scorch after a sudden change in light exposure
- Potassium deficiency, often showing as necrosis along the leaf edges
What to Check
- Spot shape — concentric rings, evenly round, or irregular and water-soaked
- Where it started — lower leaves first, or scattered throughout
- Any coating on the underside of the leaf
- How fast it's spreading — within days, or staying stable
Common Questions
What does it mean if the spots are light brown instead of dark?
Color alone isn't a reliable way to tell these apart — a lighter, tan tone shows up across several different causes, not just one. Septoria leaf spot typically has a lighter center inside a darker border, early blight's concentric rings often include lighter bands, and even late blight can look more brown than water-soaked once a lesion ages, despite starting out darker. The shape of the spot and how quickly it's spreading say more than shade on its own — a close-up photo makes it much easier to tell these apart.
Are brown spots on tomato leaves always a disease?
No. Sun scorch after a sudden change in light exposure and potassium deficiency can both cause brownish spotting or necrosis without any pathogen involved. The giveaway is usually pattern and location — scattered, ring-shaped, or water-soaked spots suggest a fungal cause, while browning concentrated at leaf edges or tied to a recent light change points elsewhere.
Should I remove tomato leaves with brown spots?
Removing heavily affected lower leaves can slow the spread of a fungal issue and improve airflow around the plant, but it isn't a fix on its own if the underlying cause isn't addressed. Avoid composting affected foliage, and it's worth confirming what's actually causing the spots before deciding on treatment.
Sources
This guide was checked against the public references below. It is general background, not a diagnosis of a specific plant.
- University of Nebraska–Lincoln Extension — publication EC1864
- UConn Integrated Pest Management — Septoria Leaf Spot on Tomatoes
- Colorado State University Extension — Tomato Early Blight (GardenNotes 718)
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