Brown or Black Spots on Tomato Leaves: What It Means and What to Do
Set your conditions — the recommendation updates instantly.
GardenSays
Can't reliably tell Early blight (Alternaria), Late blight (Phytophthora), and Septoria leaf spot apart from what you've described — they need different responses, so check the signs below before treating.
- Status
- Needs closer look
- Leading possibilities
- 3
- Causes checked
- 4
On brown or black spots, 3 possible causes matched your conditions closely enough that we can't confidently rank one above the others: Early blight (Alternaria), Late blight (Phytophthora), Septoria leaf spot.
- Late blight (Phytophthora) is one of the possibilities and can't be cured — rule it out first: Large greasy gray-green-to-brown blotches, white fuzz under leaves in humidity, spreads through the plant in days during cool wet weather.
- This is a symptom-pattern match from your written description, not a lab diagnosis. If the plant is collapsing over days (not weeks), treat it as the worst plausible cause until ruled out.
- A clear, well-lit photo (or a sample bag) to your county Extension office resolves this faster and more reliably than guessing from a description — especially when the possibilities need different treatment.
Do this this week
- 1.Compare your plant against each distinguishing sign above — that's how you tell Early blight (Alternaria) from Late blight (Phytophthora)
- 2.Rule out Late blight (Phytophthora) first — it's the one that can't wait: Large greasy gray-green-to-brown blotches, white fuzz under leaves in humidity, spreads through the plant in days during cool wet weather.
- 3.Still unsure? Send a photo to your county Extension office
Calculated result · 3 verified sources · Checked 2026-07-15 · How we decide
Why
Early blight (Alternaria) (act soon): Brown spots up to ½ inch with concentric target rings, yellow halo, starting on oldest leaves. → Remove affected leaves, mulch against soil splash, water at the base, rotate next year. Fungicide if spreading fast in wet weather.
Late blight (Phytophthora) (can't be cured): Large greasy gray-green-to-brown blotches, white fuzz under leaves in humidity, spreads through the plant in days during cool wet weather. → Act fast: remove and bag the plant (don't compost) — late blight wipes out tomatoes and potatoes garden-wide.
Septoria leaf spot (act soon): MANY small (⅛ inch) circular spots with dark borders and light gray centers — smaller and more numerous than early blight. → Same playbook as early blight: sanitation, mulch, airflow, base watering; fungicide in persistent wet spells.
- A symptom tree can only narrow the field using the conditions it asks about — when two or more causes share the same pattern for those conditions, naming a single winner would be a guess dressed up as an answer.
- The safe move when possibilities diverge this much is to rule out the worst case first, not to assume the mildest one.
When this doesn't apply
- If the plant is collapsing over days (not weeks), treat it as the most severe possibility above until ruled out.
- Multiple symptoms at once usually means today's top matches plus plain stress — fix watering first, then re-check.
How this was calculated
- 1. SymptomSource guidance
Tomato: brown or black spots — 4 known causes evaluated (extension-sourced)
- 2. Your conditionsCalculated result
Location on plant: lower; soil: moist. Causes whose known pattern matches these conditions rank higher — this narrows the list, it doesn't identify anything.
- 3. 1. Early blight (Alternaria)match score 6Calculated result
Distinguishing signs: Brown spots up to ½ inch with concentric target rings, yellow halo, starting on oldest leaves.
- 4. 2. Late blight (Phytophthora)match score 6Calculated result
Distinguishing signs: Large greasy gray-green-to-brown blotches, white fuzz under leaves in humidity, spreads through the plant in days during cool wet weather.
- 5. 3. Septoria leaf spotmatch score 5Calculated result
Distinguishing signs: MANY small (⅛ inch) circular spots with dark borders and light gray centers — smaller and more numerous than early blight.
- 6. 4. Bacterial spot/speckmatch score 3Calculated result
Distinguishing signs: Many tiny dark specks, sometimes with yellow halos, after warm rains; fruit may show raised specks too.
Data sources
- University of Minnesota Extension — Growing tomatoes in home gardens (common problems) (checked 2026-07-15)
- UMN Extension — What's wrong with my plant? (tomato diagnostic) (checked 2026-07-15)
- Cornell Vegetables — Disease factsheets (checked 2026-07-15)
What changes this answer
Where on the plant?
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People also ask
How do I diagnose other tomato problems?
Can't reliably tell Early blight (Alternaria), Late blight (Phytophthora), and Septoria leaf spot apart from what you've described — they need different responses, so check the signs below before treating. Calculate yours →
Am I watering correctly?
Water deeply 2× per week, about 1.6 gallons per plant each time. Calculate yours →
Could it be a feeding problem?
Usually nothing now — wait until when the first fruits reach about 1 inch (golf-ball size) (one catch-up feeding is fine if you never fed at planting). Calculate yours →
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