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Plant Diseases

Tomato Blight: Telling Early From Late, and What to Do

Late blight destroys a plant in days and there is no cure. How to identify it against early blight, act fast enough to save the fruit, and prevent it next year.

By The Grow In Backyard Team 4 min read Save to Pinterest — opens Pinterest in a new tab
Tomatoes on the vine covered in the greasy brown patches of late blight
Photo: Scot Nelson from Honolulu, Hawaii, USA, CC0, via Wikimedia Commons
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There are two diseases called blight, and confusing them wastes the only thing that matters with the serious one: time.

Early blight is a nuisance you manage for weeks. Late blight can take a healthy plant to dead in three to five days, and once you can see it there is nothing to spray that will stop it.

Telling them apart

Early blight against late blight
Early blight (Alternaria)Late blight (Phytophthora)
Starts onOldest, lowest leavesAnywhere, often upper leaves
LesionBrown, concentric rings, target-likeGreasy grey-green, then brown-black
MarginYellow haloPale green or water-soaked halo
UndersideNothing distinctiveWhite fuzzy growth in humid weather
StemsDark sunken lesionsDark brown-black patches
FruitLeathery dark patch at the stem endFirm brown greasy blotches, spreading
SpeedWeeks3–5 days
WeatherWarm and humidCool, wet, humid — nights around 50–60°F
OutcomeReduced cropTotal loss

The two most reliable tells: rings mean early blight, and speed means late blight. If a plant looked fine on Monday and is collapsing on Thursday, it is late blight.

Late blight is the potato famine pathogen

Phytophthora infestans is the organism that caused the Irish potato famine, and it moves through tomatoes and potatoes with the same ferocity. It spreads on wind-blown spores over considerable distances, so an outbreak in a neighbouring garden or a nearby field genuinely is your problem too. This is why acting within a day matters.

What to do about late blight

There is no treatment once it is present. The response is containment and salvage:

  1. Remove affected plants entirely, on the day you find it. Do not try to prune it out — by the time lesions show, the spores are already spread through the plant.
  2. Bag them and bin them, or burn them. Do not compost — a domestic heap does not get hot enough reliably.
  3. Harvest every firm, completely unmarked green fruit. Wash, dry, and ripen indoors in a paper bag or on a windowsill. This usually rescues a real portion of the crop.
  4. Discard anything with a mark or a soft patch, however small.
  5. Wash your hands and disinfect tools before touching other plants.
  6. Lift nearby potatoes if their foliage is affected — cut the tops off, wait two to three weeks, then harvest, as covered in growing potatoes.

Do not can or preserve blighted tomatoes

Blight raises the pH of the flesh. Standard water-bath canning recipes rely on tomatoes being acidic enough to prevent botulism, and blighted fruit can fall outside that assumption. Eat clean fruit fresh, or freeze it — do not preserve it by any method that depends on acidity.

What to do about early blight

Much less dramatic. The plant will usually crop through it.

  • Remove affected lower leaves as they appear, and any leaf touching the soil.
  • Mulch — early blight spores live in soil and splash upward onto foliage. Two to three inches of mulch cuts that off, and it is the single most effective step.
  • Water at the base, never overhead.
  • Feed and keep the plant vigorous. A strong plant outgrows the damage; a stressed one does not.
  • Improve airflow by removing crowded growth.

Prevention, which is where the work actually is

Everything that keeps foliage dry and air moving:

Spacing. Crowding is the root cause of most fungal problems on tomatoes. Two to three feet between indeterminate plants.

Watering. At the base, in the morning, so any splash dries by evening. Drip or a can with the rose off, never a sprinkler.

Mulch. Stops soil splash, which is the main infection route for early blight.

Pruning. Remove the lowest 12 inches of leaves once plants are established, and side shoots on indeterminate types. Air needs to move through the plant.

Rotation. Three to four years before tomatoes or potatoes return to the same ground. Early blight persists in soil and debris; this matters.

Clear debris in autumn. Take out every stem, leaf and fallen fruit at the end of the season, and remove volunteer potato plants the following spring — those are the classic late blight reservoir.

Resistant varieties. Crimson Crush, Mountain Magic, Defiant, Iron Lady. Resistance is a large advantage, not a guarantee.

Grow under cover if you can. A greenhouse or polytunnel keeps rain off the foliage entirely, and in a wet climate that is the most reliable protection against late blight there is.

A quick diagnostic

If it is not blight, it may be simpler than you fear:

  • Lowest leaves yellowing, no spots — usually nitrogen, or just age.
  • Leaves curling upward, plant otherwise healthy — usually physiological leaf roll.
  • White powdery coating — powdery mildew.
  • Dark sunken patch on the fruit base only — blossom end rot, a watering problem.
  • Small dark spots with yellow halos on lower leaves, no rings — Septoria leaf spot; treat as for early blight.

Common questions

What is the difference between early and late blight?

Early blight starts on the lowest, oldest leaves as brown spots with concentric rings like a target, and moves slowly up the plant over weeks — it is disfiguring but survivable. Late blight appears anywhere, as greasy grey-green patches that turn brown-black with a pale halo, often with white mould underneath in humid weather, and it can kill a plant in three to five days.

Can you treat tomato blight once you see it?

There is no cure for late blight. Remove and destroy affected plants immediately to protect the rest, and harvest any firm, unmarked green fruit to ripen indoors. Early blight can be managed by removing affected lower leaves, mulching, improving airflow and feeding — the plant usually crops despite it.

Can you eat tomatoes from a plant with blight?

Fruit that is firm and completely unmarked is safe to eat and can be ripened indoors. Discard anything with brown or greasy patches, and anything soft. Do not attempt to preserve or can blighted fruit — the disease raises the pH of the flesh, which makes standard canning recipes unsafe.

Does blight stay in the soil?

Early blight does — its spores survive on infected debris and in soil for a year or more, which is why rotation and clearing debris matter. Late blight does not survive winter on its own in most climates; it overwinters in living tissue such as infected potato tubers left in the ground, and blows in on the wind from elsewhere.

How do you prevent tomato blight?

Airflow and dry leaves, above everything. Space plants generously, prune lower leaves, water at the base rather than overhead and in the morning, mulch to stop soil splashing onto foliage, rotate crops, and grow resistant varieties. Under cover — a greenhouse or polytunnel — is the most reliable protection against late blight in a wet climate.

What are the most blight-resistant tomato varieties?

Crimson Crush, Mountain Magic, Defiant, Iron Lady and the Cherokee-derived resistant hybrids have strong late-blight resistance. Resistance is not immunity — in a bad year with prolonged warm wet weather they can still be hit — but a resistant variety often crops through conditions that destroy a standard one.

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