Brown Spots on Potato Leaves: Early Blight or Late Blight?

Set your conditions — the recommendation updates instantly.

GardenSays

Can't reliably tell Early blight (Alternaria) and Late blight (Phytophthora) apart from what you've described — they need different responses, so check the signs below before treating.

Status: Needs closer look
Leading possibilities: 2
Causes checked: 2

On brown or black spots on leaves, 2 possible causes matched your conditions closely enough that we can't confidently rank one above the others: Early blight (Alternaria), Late blight (Phytophthora).

Late blight (Phytophthora) is one of the possibilities and can't be cured — rule it out first: Large brown blotches with a green-gray edge, spreading fast in cool damp weather — can take down the whole plant and infect the tubers.
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 brown blotches with a green-gray edge, spreading fast in cool damp weather — can take down the whole plant and infect the tubers.
3.Still unsure? Send a photo to your county Extension office

Calculated result · 3 verified sources · Checked 2026-07-17 · How we decide

The link reopens this exact calculation.

Why

Early blight (Alternaria) (act soon): Round brown spots on older, lower leaves first, up to 1/2 inch, with target-like concentric rings as they enlarge. → Remove affected leaves, mulch to stop soil splash, improve airflow; fungicide only if it keeps advancing.

Late blight (Phytophthora) (can't be cured): Large brown blotches with a green-gray edge, spreading fast in cool damp weather — can take down the whole plant and infect the tubers. → No cure. Remove and destroy affected plants immediately to protect the rest of the patch and the tubers; don't compost.

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
Potato: brown or black spots on leaves — 2 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 4Calculated result
Distinguishing signs: Round brown spots on older, lower leaves first, up to 1/2 inch, with target-like concentric rings as they enlarge.
4. 2. Late blight (Phytophthora)match score 3Calculated result
Distinguishing signs: Large brown blotches with a green-gray edge, spreading fast in cool damp weather — can take down the whole plant and infect the tubers.

Data sources

University of Minnesota Extension — Early blight in tomato and potato — Small dark spots form on older foliage near the ground; leaf spots are round, brown, up to 1/2 inch, with target-like concentric rings on larger spots. One of the most common potato diseases. (checked 2026-07-17)
University of Minnesota Extension — Late blight of tomato and potato — Potentially devastating; infects leaves, stems and tubers. Leaf infections are large brown blotches with a green-gray edge. Favors cool (60-70F), damp conditions and can cause total crop failure if untreated. (checked 2026-07-17)
University of Minnesota Extension — What's wrong with my plant? (potato tuber diagnostic) — Potato scab is a bacterial disease causing dark, rough, corky spots on the skin of the tuber but does not rot them — ugly but completely edible. (checked 2026-07-17)

What changes this answer

Where on the plant?

Soil right now

Every change updates the answer, the math, the weekly steps and the share link instantly.

Your next decisions

Potato Scab

Diagnose potato scab — is it still safe to eat, and how to reduce it next season