Tomato Blight: How to Identify and Treat Late Blight

There is a particular sinking feeling that arrives in a warm, wet late summer: you go out to your tomatoes and yesterday's healthy plant now has dark, water-soaked patches racing across the leaves. That speed is the signature of late blight, the same disease that caused the Irish potato famine, and it can flatten a row of tomatoes in under a week. Knowing what you are looking at — and acting the same day — is the difference between losing one plant and losing the lot.

What late blight is (and why it moves so fast)

Late blight is caused by Phytophthora infestans, a fungus-like water mould rather than a true fungus. It thrives in exactly the conditions a British or northern-hemisphere late summer often provides: mild temperatures of roughly 10–24°C combined with long spells of leaf wetness from rain, dew or humidity. Under those conditions the spores germinate on wet foliage within hours and are carried on the wind for miles, which is why blight can appear in a garden with no obvious source. It attacks tomatoes and potatoes, the two most susceptible members of the nightshade family.

How to identify late blight

Blight starts on the leaves and moves down and inward. Look for these signs in order of appearance:

• Irregular, greasy-looking grey-green to brown patches on leaves, often starting at the tips or edges, that spread quickly rather than staying as neat spots.
• A fine white or greyish fuzzy growth on the underside of affected leaves in humid weather or early morning — this is the mould sporulating, and it is the clearest confirmation.
• Dark brown to black streaks or lesions on the stems and leaf stalks, which can girdle and collapse whole sections of the plant.
• On the fruit: firm, greasy brown or bronze blotches, usually on the shoulders, that stay hard rather than turning soft and watery — the flesh underneath is discoloured and inedible.
• Rapid decline: whole leaves brown, curl and die within a day or two, and in wet weather the plant can smell faintly of rot.

Late blight vs early blight — an important distinction

Gardeners often use 'blight' for any browning tomato leaf, but the two main diseases need different responses. Early blight (Alternaria) is the slower, more common one: it produces dark brown spots with concentric rings, like a tiny archery target, usually on the older lower leaves first, often ringed by a yellow halo. It is unsightly and weakens the plant but rarely kills it outright, and prompt removal of affected leaves plus better airflow usually keeps it in check. Late blight is the emergency: no target rings, no slow start on the bottom leaves — instead large greasy patches that spread across the whole plant fast, plus that tell-tale white mould underneath and hard brown patches on the fruit. If it is racing and greasy, treat it as late blight.

What to do the moment you spot it

There is no spray that cures a plant once late blight has taken hold, so the goal shifts to containment. Move quickly:

• Remove affected plants promptly. If only a leaf or two is hit you can cut them off well below the visible damage, but if patches are spreading over the plant, pull the whole thing.
• Bag and bin it, or burn it — never compost blighted material. Home compost heaps rarely get hot enough to kill the spores, and blight overwinters on living plant tissue and tubers.
• Do not leave prunings or fallen leaves on the soil, and clear any self-sown potato or tomato volunteers nearby, as they harbour the disease.
• Harvest what you can. Firm, unblemished green fruit from an affected plant can be picked and ripened indoors on a windowsill, away from the infected foliage — check it daily and discard any that develops brown patches.
• Disinfect secateurs and wash your hands before touching healthy plants, as spores spread on tools and skin.

Warning: never eat tomatoes showing blight lesions, and do not try to save affected fruit by cutting the bad parts away — the mould penetrates deeper than the visible blotch. Only fully firm, unmarked fruit is worth ripening on.

How to prevent late blight next season

Because blight blows in on the wind, you cannot guarantee immunity, but you can make your plants a far harder target. Keep the foliage dry: water the soil, not the leaves, in the morning so any splashes dry fast, and grow tomatoes under cover — a greenhouse, polytunnel or even a simple rain cover keeps leaves dry and dramatically lowers the risk. Space plants generously and remove lower leaves and side shoots so air moves freely through the canopy. Grow blight-resistant varieties, which are widely available and bred specifically for wet-summer regions. Rotate where you grow tomatoes and potatoes so the soil has a break, and clear every scrap of the old crop at the end of the season. Where blight is a recurring problem, a preventative copper-based fungicide applied before symptoms appear can slow its establishment, though it protects rather than cures — check the current label for edible-crop approval in your country.

Not certain whether those brown patches are late blight, early blight, a nutrient issue or simple sun and heat damage? Photograph an affected leaf against a plain background in PlantMe. The app identifies the plant and checks the symptoms against a library of over 500 diseases and disorders, so you can tell a killer disease from a cosmetic one — and act fast enough to matter — in seconds.