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.
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.
Want a real answer for your plant?
The causes above are a starting point, not a diagnosis — they don't replace looking at your actual plant. Upload a photo for an analysis based on the real symptoms.