Brown spots on tomato plants: causes and fixes
The most common causes of brown spots on tomato plants are early blight (*Alternaria solani*), Septoria leaf spot (*Septoria lycopersici*), bacterial speck (*Pseudomonas syringae*), bacterial spot (*Xanthomonas vesicatoria*), and late blight (*Phytophthora infestans*). Early blig
Brown spots on tomato foliage are one of the most common mid-season distress signals, and they're tricky because several diseases look similar at first glance. The spot's shape, size, edge pattern, and where it appears on the plant are the diagnostic keys.
Quick diagnostic table
| Spot description | Plant location | Most likely cause |
|---|---|---|
| Brown spots with concentric rings (target pattern), yellow halo | Lower leaves first | Early blight (Alternaria) |
| Small (1/4 in) circular spots, gray-white center, dark border | Lower leaves first | Septoria leaf spot |
| Very small dark brown-black spots, water-soaked at first | Any location, leaves and fruit | Bacterial speck |
| Dark brown to gray-green water-soaked patches, spreading rapidly | Any location | Late blight (Phytophthora) |
| Brown, sunken spot on blossom end of fruit | Fruit only | Blossom end rot (calcium disorder) |
| Large irregular brown patches, no distinct spots | Upper/lower leaves | Bacterial canker, magnesium deficiency |
| Brown streaks inside stem | Cut stem base | Fusarium/verticillium wilt |
Cause 1: Early blight — the most common cause
Per NC State Extension, early blight is "the most common and economically important foliar disease of tomato in the US." It's caused by the fungus Alternaria solani and affects leaves, stems, and fruit.
What it looks like: Brown to dark brown spots, 1/4 to 1/2 inch in diameter, with distinctive concentric rings that create a "target board" or "bull's-eye" appearance. The spots are surrounded by a yellow halo. Per NC State Extension, "the concentric rings are diagnostic for early blight."
Where it starts: Lower, older leaves first — the disease progresses up the plant from the bottom over the course of weeks.
Conditions that favor it: Per Penn State Extension, "warm temperatures (75–85°F) and high humidity" favor early blight. The fungal spores survive in infected plant debris in the soil and are dispersed by rain splash and irrigation.
How to fix:
- Remove infected lower leaves as soon as you spot the first signs
- Apply a copper-based or chlorothalonil fungicide per label rates — per NC State Extension, "applications every 7–10 days during wet weather are necessary"
- Mulch the soil surface heavily (3 inches minimum) to prevent rain splash of spores from soil to leaves
- Water at the base of the plant; avoid wetting foliage
- Rotate tomatoes to a new bed location each season — the fungus overwinters in soil and infected debris
Resistant varieties: Several tomato varieties have good early blight resistance including 'Iron Lady', 'Mountain Magic', and 'Defiant'. Per Penn State Extension, "planting resistant varieties is the most effective long-term management strategy."
Cause 2: Septoria leaf spot
Septoria leaf spot is almost as common as early blight in the eastern US. Per Cornell Cooperative Extension, "Septoria leaf spot and early blight often occur together on the same plant."
What it looks like: Small (1/8 to 1/4 inch), circular spots with grayish-white centers and dark brown borders with a yellow halo. Per Cornell, "the grayish-white center with dark border is the key diagnostic feature distinguishing Septoria from early blight." With a hand lens, you can often see tiny black dots in the center of older lesions — these are the pycnidia (fungal fruiting bodies).
Management: Same as early blight — remove infected leaves, copper or chlorothalonil fungicide, mulch, base watering. Per Cornell Cooperative Extension, "the two diseases respond to the same fungicide program."
Cause 3: Bacterial speck (Pseudomonas syringae pv. tomato)
Bacterial speck produces very small (1/16 inch), dark brown to black spots, often with a narrow yellow halo. Per NC State Extension, "the spots are much smaller than those of early blight or Septoria." Bacterial speck affects leaves, stems, and fruit — fruit shows small, raised dark spots with sunken centers.
Conditions that favor it: Per UC IPM, "bacterial speck is most severe under cool, wet conditions early in the season." Temperatures of 60–75°F and wet foliage are ideal.
How to confirm: The spots are tiny (pin-head to small pencil eraser size), dark, and often appear in large numbers across the leaf surface.
How to fix: Per UC IPM, "copper-based bactericides provide some control when applied before disease establishment." Unlike fungal diseases, copper on an established bacterial infection has limited effect — prevention and copper applications at the start of wet weather are more useful than treatment. Remove severely infected plant material.
Cause 4: Bacterial spot (Xanthomonas vesicatoria)
Similar to bacterial speck but with slightly larger, more angular spots. Per NC State Extension, "bacterial spot causes irregularly shaped, dark brown lesions on leaves, stems, and fruit." Fruit lesions are dark, raised, scabby, and sunken — a key distinction from fungal diseases.
Conditions: Warm, wet weather (75–85°F) — warmer than bacterial speck. Per NC State Extension, "bacterial spot is most severe in warm, wet conditions."
Management: Copper bactericide applications. Per NC State Extension, "copper compounds mixed with mancozeb are more effective than either alone."
Cause 5: Late blight (Phytophthora infestans)
Late blight is the most destructive tomato disease. It caused the Irish Potato Famine in the 1840s and can kill a tomato planting in days under the right conditions.
What it looks like: Per Penn State Extension, late blight appears as "large, irregular, water-soaked patches that quickly turn brown to olive-green and develop a white, downy mold on the undersides of affected leaves in humid conditions." The affected tissue looks more like it's rotting than spotting.
Conditions: Per Penn State Extension, "cool, wet conditions (60–70°F, sustained humidity above 90%)" favor late blight. It spreads extremely rapidly under these conditions.
How to confirm: Look for the white sporulation on the underside of affected tissue during humid mornings. The rapid spread is also diagnostic.
How to fix: Per Penn State Extension, "once late blight is established in a planting, it is very difficult to control." Protective fungicide applications (chlorothalonil, copper + mancozeb) must be applied before infection. If a planting is heavily infected, removing and bagging all plant material prevents spread to neighboring gardens — late blight spores can travel miles in wind. Do not compost infected plants.
Cause 6: Blossom end rot
This is not a disease but a physiological disorder — brown, leathery, sunken areas on the blossom end (bottom) of tomato fruit.
Cause: Per Penn State Extension, "blossom end rot is caused by calcium deficiency within the fruit, usually due to irregular soil moisture affecting calcium uptake" rather than soil calcium levels. The soil may have adequate calcium, but drought stress or irregular watering prevents uptake.
How to fix: Per Penn State Extension, "consistent soil moisture management — not adding calcium sprays — is the primary fix." Mulch heavily; maintain consistent watering. Foliar calcium sprays provide minimal benefit per most Extension research when soil moisture is the underlying cause.
See our full article on blossom end rot for more detail.
Cause 7: Walnut toxicity
If your tomato is planted near or downwind from a black walnut tree (Juglans nigra), the allelopathic compound juglone from walnut roots can cause wilting and browning of tomato foliage. Per Penn State Extension, "tomatoes are among the most sensitive plants to juglone toxicity." Symptoms include browning, wilting, and eventual death.
Prevention strategies: the foundation
Per Penn State Extension, "cultural practices are the foundation of tomato disease management." The three most effective preventive measures:
- Crop rotation: Move tomatoes to a different bed each year. Most fungal and bacterial pathogens survive in soil and on debris — rotation breaks the cycle. Per Penn State Extension, "a 3-year rotation out of solanaceous crops (tomatoes, peppers, eggplant) is ideal."
- Mulching: Apply 3 inches of straw or shredded bark mulch over the root zone. Per NC State Extension, "mulch reduces splash dispersal of soilborne pathogens to lower leaves."
- Watering technique: Drip irrigation or soaker hoses keep foliage dry. Per UC IPM, "wet foliage for extended periods significantly increases infection risk."
Common mistakes
| Mistake | What happens | Fix |
|---|---|---|
| Applying fungicide after late blight is established | Limited effectiveness | Copper sprays protect; they don't cure established late blight |
| Overhead watering | Spreads spores; wet foliage prolongs infection periods | Switch to soaker hoses or drip |
| No mulch | Rain splash continuously reinfects lower leaves | 3-inch mulch reduces this dramatically |
| Composting infected plant debris | Disease survives; inoculates compost | Bag and dispose; do not compost |
Frequently asked
Can I eat tomatoes from a plant with early blight?
Yes. Per Penn State Extension, "early blight on leaves does not affect fruit safety." Tomatoes from blighted plants are edible. You may see some stem-end or calyx damage from the fungus, but this is cosmetic.
What's the most reliable way to prevent tomato diseases?
Per NC State Extension, "choosing resistant varieties, rotating crops, and applying preventive fungicides when conditions favor disease" is the most effective approach. No single strategy is sufficient — all three together provide the best protection.
Is copper fungicide safe for organic gardening?
Copper is approved for organic production per USDA National Organic Program standards. Per UC IPM, "copper-based products are allowed in certified organic production." However, copper accumulates in soil with repeated applications — per Penn State Extension, "copper should be used judiciously and rotated with other approved products to avoid soil accumulation."
Sources
- NC State Extension — Early Blight of Tomato
- Penn State Extension — Tomato Disease Management
- UC IPM — Tomato Diseases
- Cornell Cooperative Extension — Septoria Leaf Spot
Sources
- 1. NC State Extension — Early Blight of Tomato
- 2. Penn State Extension — Tomato Disease Management
- 3. UC IPM — Tomato Diseases
- 4. Cornell Cooperative Extension — Septoria Leaf Spot