Black Spots Spreading Fast on Leaves? Diagnose It Right
Dark spots showed up on your leaves a few days ago. Now there are more of them — and the plant looks worse every morning.
Your gut says disease. Your gut is probably right. But which one? Five different problems make dark spots that spread, and they demand very different responses. Spraying for the wrong one wastes your week; ignoring the right one can cost you the plant.
Here's our honest take: if the weather's been cool and wet and the spots are racing, this looks most like late blight — but early blight or bacterial spot is also possible, especially if it's been warm. Diagnosis is a process, not a verdict. Work the clues below in order and you'll land on the right answer.
In this guide, we'll start with the single fastest test, rank the five suspects from most to least likely, and give you an IPM-first action plan for whatever you find.
The Flip Test: Your Fastest Clue
Before anything else, do this: on a humid morning — or right after rain — flip over a freshly spotted leaf and look at the underside.
If you see a white, fuzzy growth ringing the spots, especially along the lesion edges, that's the signature of late blight sporulating. Combined with dark lesions on the stems, it's the strongest field clue there is. Our full late blight guide walks through the complete picture from there.
Two cautions, because honesty matters here. First, the fuzz only shows in humid air — on a dry afternoon you won't see it even on a truly infected plant, so no fuzz doesn't mean no late blight. Second, other diseases leave their own underside signs (septoria's black dots, for instance), so keep reading before you decide. A small hand lens makes this check much easier — our gardening tools hub rounds up the simple gear worth keeping in your pocket.
The Ranked Lineup: Five Suspects, Most Likely First
Ranked for the classic complaint — dark spots, spreading fast. One big caveat: this ranking assumes recent cool, wet weather. A heat wave reshuffles the deck toward early blight and bacterial spot, as you'll see below.
| Rank | Suspect | What the spots look like | Speed of spread | Weather link | The one test that separates it |
|---|---|---|---|---|---|
| 1 | Late blight (Phytophthora infestans) | Water-soaked dark spots starting at leaf edges or tips, turning brown to purplish-black; not stopped by leaf veins; stems and fruit can be hit too | Fastest — visible damage in days, whole plants in a couple of weeks | Cool and wet: around 60–70°F with leaf wetness | Flip the leaf: white fuzzy growth on the underside in humid air — plus dark stem lesions — points to late blight |
| 2 | Early blight (Alternaria solani) | Concentric target rings — a bull's-eye of brown bands, usually ¼–½ inch across, often with a yellow halo; starts on older, lower leaves | Steady — climbs the plant over weeks | Warm and humid: 75–85°F | Target rings mean early blight. No rings? It's probably not early blight |
| 3 | Bacterial spot or speck (Xanthomonas spp. / Pseudomonas syringae pv. tomato) | Small dark spots, about ⅛ inch, sometimes with yellow rings; speck runs tinier; can speckle fruit with small raised spots; never any fuzz | Moderate — spreads in wet weather over weeks | Warm and wet: 75–86°F | Spots stay small and never grow fuzz; fruit spots are tiny, not big greasy blotches |
| 4 | Septoria leaf spot (Septoria lycopersici) | Small round spots, 1/16–1/8 inch, with tan-gray centers and dark margins; tiny black dots visible in the centers with a hand lens; fruit stays clean | Can move fast in long wet spells, but climbs from the bottom up | 68–77°F and wet | Hand-lens black dots in tan centers, plus clean fruit, says septoria |
| 5 | Sunscald | Bleached, papery, pale tan patches — not dark spots — on sun-exposed fruit and leaves | Not infectious; appears after foliage loss or sudden sun exposure | Hot, intense sun | Bleached, not dark, and only where the sun hits; nothing actually spreads |
Why the ranking can shift. Weather is the tiebreaker. Cool and wet — 60s, rainy, heavy dew — pushes late blight to the top. Hot and humid — 80s — favors early blight and bacterial spot. Spots confined to the lowest leaves with tan centers and black dots, climbing slowly upward, say septoria even when they're spreading steadily. And bleached, papery patches where the sun beats down, with no spreading at all, are sunscald, full stop.
And remember, these temperature windows shift with your climate region — a cool coastal summer can run late-blight weather for months, while a hot interior may only see it after unusual rain.
So: this looks most like late blight when spots are dark, water-soaked, and fast, and the weather's been cool and damp. But early blight is also possible — the target rings will tell you. When in doubt, our early blight vs late blight comparison puts the two side by side, and brown spots on tomato leaves covers the early-blight symptom angle in more depth.
What To Do Right Now
Found your suspect? Here's the IPM order — whatever the diagnosis turns out to be, these steps help.
Don't touch wet plants. Handling wet foliage spreads nearly every one of these diseases. Scout when things are dry — our scouting guide shows where problems hide first.
Remove the worst leaves. Strip badly spotted leaves into a bag — do it when the plant is dry — and put the bag in the trash, not the compost. Wash your hands and tools before touching healthy plants.
Dry the canopy. Water at the base in the morning so leaves dry fast. Space and stake tomatoes for airflow, and mulch to stop soil splash.
Protect what's left — as a last resort. If late blight is confirmed nearby and cool, wet weather continues, protectant sprays (generic chlorothalonil or copper-based classes) applied before symptoms and reapplied every 5–7 days can slow it. Read the label, and know that no spray guarantees a healthy crop.
Plan next season. Look for varieties with labeled blight resistance — our resistant tomato varieties guide explains what the labels mean — and rotate nightshades to fresh ground. Starting clean is half the battle: browse our tomato seeds and the wider vegetable collection when you plan.
Frequently Asked Questions
I see spots but no white fuzz — can it still be late blight?
Yes — and this is the most common reason gardeners rule it out wrongly. The fuzz only forms in humid air, so check on a dewy morning. You can also tuck a suspect leaf in a closed container with a damp paper towel overnight and look again in the morning. Still nothing? Then weigh the other suspects.
The spots are spreading, but it's been hot — is late blight still likely?
Less likely. Late blight wants it cool, around 60–70°F; sustained heat above about 95°F stalls it. In hot weather, early blight and bacterial spot move up the suspect list. This looks most like one of those — but keep watching if the weather cools.
Will the disease come back next year?
It depends which one. Late blight can't overwinter in bare soil, on stakes, or in seed — it needs living tissue like volunteer potatoes or cull piles. The other suspects can linger on old plant debris, so a fall cleanup and rotation help regardless.
Can I compost the spotted leaves?
No — bag them and trash them. Disease spores can keep spreading from debris in the pile.
Are the tomatoes safe to eat?
These pathogens attack plants, not people — but don't eat rotting or badly spotted fruit. Late blight fruit rots from the inside out; when in doubt, throw it out.
Sources
- Cornell Cooperative Extension
- University of Massachusetts Extension
- University of Georgia Extension
- Penn State Extension
- University of Delaware Extension
Wrapping Things Up!
Dark spots spreading fast are scary — but they're also talkative. They tell you about the weather, the speed, and with one flipped leaf, sometimes the answer itself.
Work the clues, act in IPM order, and don't guess alone: if a spot has you stumped, reach out — we're happy to help you figure it out. Happy gardening!
