Garden
Plant Problems
Tomato Blight: One Name, Two Diseases, and Opposite Instructions
Look for rings. Small brown spots with target-like rings inside them, starting on the lowest leaves, are early blight — a season-long nuisance you manage. Large dark blotches with a gray-green edge, anywhere on the plant including new growth, are late blight, which can kill a planting in days and spreads to other gardens.
Look for rings, and look at which leaves
Two questions separate them, and you can answer both standing at the plant.
Do the spots have rings inside them? Minnesota on early blight: "Larger spots have target-like concentric rings. The tissue around spots often turns yellow." That target pattern is the single most reliable sign, and late blight does not make it. Late blight's lesions are "large, dark brown blotches with a green-gray edge" — a blotch with a soft margin, not a bullseye.
Where on the plant is it? Early blight starts at the bottom and works up: "Initially, small dark spots form on older foliage near the ground." Late blight is anywhere, including new growth at the top. A young leaf at the top of the plant with a big dark blotch on it is the combination that should worry you.
One more if the weather is muggy: late blight in high humidity grows "thin, powdery white fungal growth" on the leaf. Early blight never does.
The two diseases, side by side
Everything here is from University of Minnesota Extension's two pages, one for each disease — the same publisher, so the contrast is theirs rather than assembled by us.
| Early blight | Late blight | |
|---|---|---|
| What it is | A fungus — Alternaria tomatophila and Alternaria solani | A WATER MOLD, Phytophthora infestans. Not a fungus at all, which is why it behaves so differently |
| The spots | "Target-like concentric rings", with the tissue around them often turning yellow | "Large, dark brown blotches with a green-gray edge" — no rings |
| Where it starts | "Small dark spots form on older foliage near the ground" — bottom up | Anywhere, including young leaves at the top |
| The fruit | "Leathery and black, with raised concentric ridges" | "Firm, dark brown, circular spots develop and cover large parts of the fruit" |
| Speed | Spots in as little as five days; severe defoliation over a season | Thousands of spores per lesion in under five days, and "total crop failure if untreated" |
| Where it spends the winter | Right where you are — "in infected plant debris and soil", and on seed and transplants | Mostly NOT in your garden. It survives only "if protected in potato cull piles"; otherwise it arrives windblown from the south or on shipped-in transplants |
| Whose problem it is | Yours, over seasons | The neighborhood's, this week |
The row that changes what you do
The last two rows are the page. Early blight lives in your soil and comes back, so it is a rotation, mulch and resistant-variety problem you chip away at year on year. Late blight mostly is not in your soil at all — it blows in — so resting the bed does nothing, and the useful response is immediate rather than cultural.
And Minnesota's instruction for late blight is not a product. It is to contact the extension team, because "late blight is an important region-wide issue and neighboring crops are at risk". That is an unusual thing for an extension page to ask of a home gardener, and it is the clearest signal available that this half of the name is not an ordinary garden problem.
What to do, once you know which
The first two steps are the same for both. The last two are opposites, which is the reason this page exists.
- Read the spot, then read the position. Rings inside the spot and damage starting on the lowest leaves means early blight. A big dark blotch with a soft gray-green edge, especially on young growth near the top, means late blight. White powdery growth on the leaf in humid weather confirms late blight.
- Get the infected material out, either way. Bag it and bin it rather than composting it. Early blight overwinters in debris, so leaving it on the soil is how it comes back next year; late blight is producing enormous numbers of airborne spores while it sits there.
- If it is EARLY blight, settle in for a season of management. Minnesota's controls are cultural and long-run: resistant varieties, mulch to stop soil splashing onto the lowest leaves, drip irrigation rather than overhead watering, and fungicides rotated between chemical families if you use them. Nothing here is urgent, and nothing here is finished this year.
- If it is LATE blight, act this week and tell somebody. Remove and destroy affected plants rather than nursing them, because the disease can take the whole planting and spreads on the wind. Minnesota asks growers to contact their extension team, since neighboring crops are at risk. Do not compost the plants and do not leave potato culls lying about — that is the one way it does overwinter.
The two do not exclude each other. A plant can have early blight low down in August and pick up late blight from a neighboring field the same week, so finding one does not mean you should stop looking.
If you cannot decide, treat it as late blight for a few days — the cost of being wrong is a bagged plant, and the cost of being wrong the other way is a lost planting and a spread you started.
What this page covers, and where our own data is wider than its sources
This page is about tomatoes and potatoes. Minnesota's late blight page states its host range is "typically limited to potato and tomato", and its early blight page names tomatoes, potatoes, peppers and Solanaceae weeds. Everything above is scoped to that.
Our own database is wider than that, and we think it is wrong. We attach both blights to celery, celeriac and lovage as well — three carrot-family crops that neither Minnesota page covers. Celery does get diseases called early and late blight, and they are different organisms that happen to share the common names. We have flagged it rather than quietly writing around it, and until it is checked, nothing on this page should be read as advice about celery.
We also do not tell you which fungicide to buy. Minnesota's early blight guidance is to rotate between chemical families to slow resistance, which is a real instruction and not a product recommendation, and late blight's answer is removal and a phone call rather than a spray.
Questions about tomato blight
How do I tell early blight from late blight on my tomatoes?
Two things: rings, and position. Early blight makes spots with target-like concentric rings inside them, and it starts on the oldest leaves near the ground and works upward. Late blight makes large dark brown blotches with a gray-green edge, no rings, and it appears anywhere on the plant including new growth at the top. In humid weather late blight also grows a thin white powdery layer on the leaf, which early blight never does.
Is tomato blight in my soil? Can I plant tomatoes there next year?
It depends entirely on which one you had. Early blight overwinters in infected plant debris and soil, so yes — it is there, and rotation, mulch and clearing debris all matter. Late blight is different: it survives northern winters only if protected in potato cull piles, and otherwise arrives each year windblown from the south or on transplants shipped in. Resting the bed is a sound response to early blight and does almost nothing about late blight.
Do I really need to tell anyone about late blight?
Minnesota asks growers to contact their extension team, and the reason is in their own words — late blight is a region-wide issue and neighboring crops are at risk. It is a water mold that produces thousands of airborne spores per lesion in under five days, so an untreated plant in one garden is a genuine threat to plantings nearby. It is the only problem in this section where the recommended action includes telling somebody outside your own garden.
Can I save a plant with blight?
With early blight, usually yes — it causes severe defoliation over time rather than sudden death, and mulching, watering at the base and removing affected lower leaves buy you a workable season. With late blight, no: the advice is to remove and destroy affected plants rather than nurse them, because it can cause total crop failure if untreated and every day the plant stands is more spores leaving your garden.
Sources
Every planting date, depth, and temperature on this page traces to these university extension guides. Where they differ, the ranges above hold the whole spread.
- University of Minnesota Extension — Early blight in tomato and potato
- University of Minnesota Extension — Late blight of tomato and potato
The short version: rings and bottom leaves mean early blight and a season of management. A dark blotch with a soft edge, anywhere on the plant, means late blight and a decision this week.
And if you genuinely cannot tell, act as though it is late blight for a few days. Being wrong that way costs you a plant; being wrong the other way can cost the neighborhood.