Tomato Blight: How to Spot, Stop and Prevent It

Tomato blight is a fast-moving, fungus-like disease (caused by the organism Phytophthora infestans) that thrives in the warm, humid weather of a typical British summer. You cannot cure it once it takes hold, but you can slow it down by acting early — and with a few changes, you can largely prevent it striking next year.

It is one of the most disheartening problems a beginner can meet, because a healthy-looking plant can collapse within days. The good news is that blight is predictable, and most of the defences against it are simple and cost nothing.

How to recognise it

Blight usually shows up from mid-summer onwards, often after a warm, wet spell. Catching it in the first day or two gives you the best chance of saving some fruit, so it pays to know the signs.

On the leaves. Look for irregular brown or grey-brown blotches, usually starting at the leaf tips or edges. They often have a pale, slightly yellow halo around them, and in damp conditions you may see a faint white, fuzzy growth on the underside. Within a day or two the blotches spread and the whole leaf shrivels and browns.

On the stems. Dark brown or black streaks and lesions appear on the main stem and leaf stalks. Stem infections are particularly bad news — once blight is in the stem, the plant is usually beyond saving.

On the fruit. Green or ripening tomatoes develop firm, brown, leathery patches that spread quickly, often with a slightly greasy or marbled look. The rot works inward, and affected fruit is not worth eating.

The speed is the giveaway. A few cosmetic spots that sit still for weeks are more likely something else. Blight moves fast — if blotches double overnight after warm, wet weather, treat it as blight.

Why it happens

Blight needs two things to take off: warmth and moisture. The spores travel on the wind and in rain splash, and they germinate when leaves stay wet and temperatures sit roughly between 10°C and 24°C for long enough — exactly the muggy conditions a UK summer serves up.

The Met Office and gardening bodies track these conditions as "Smith periods" — two consecutive days where the temperature stays above 10°C and humidity stays high for several hours. When a Smith period is forecast in your area, blight risk is high, and it is worth checking your plants closely and keeping the foliage as dry as you can.

A few things make an outbreak much more likely:

• Splashing water. Watering over the leaves, or heavy rain bouncing soil up onto the lower foliage, spreads spores beautifully.
• Crowded, still air. Plants packed close together stay damp for longer, which is all blight asks for.
• Nearby potatoes. This is the big one. Tomato blight is the very same organism as potato blight, and an outbreak in a potato patch is a common source of spores that then drift onto tomatoes. If you grow both, keep them well apart.

Outdoor tomatoes are far more exposed than those under cover, simply because their leaves get wet from rain and dew. That single fact explains why greenhouse and polytunnel tomatoes so often escape an outbreak that flattens the plants outside.

What to do once you see it

There is no spray a home gardener can buy to cure blight, so the aim is to slow it and rescue what you can.

Remove affected leaves straight away. At the very first sign, snip off any blotched leaves and stems with clean secateurs, cutting well back into healthy growth. Do not put them on the compost heap — bag them and bin them, or burn them, so the spores do not linger.

Improve the airflow. Strip off the lowest leaves and any congested growth so air moves freely and the foliage dries faster after rain. Drier leaves are far less hospitable to the disease.

Stop overhead watering. Water only at the base of the plant, into the soil, never over the leaves. Keeping the foliage dry is one of the most useful things you can do mid-outbreak.

Harvest any healthy fruit. If blight is spreading despite your efforts, pick every firm, unblemished tomato — green ones included. Green tomatoes will ripen indoors on a sunny windowsill, or you can pop them in a paper bag with a ripe banana to speed things along. Once the plant is clearly going down, pull it up and dispose of it (again, not on the compost) to protect any other plants nearby.

Be honest with yourself about timing. Once stems are streaked and fruit is rotting across the plant, no amount of leaf-snipping will turn it around. At that point, salvage the harvest and move on.

How to prevent it next year

This is where you can really get ahead of blight. None of these steps is difficult, and together they make a serious difference.

Grow under cover. Keeping tomatoes in a greenhouse, polytunnel or even a sheltered porch keeps the rain off the leaves, which is the single most effective defence there is. If you only have room for a few plants, growing them under cover or as tomatoes in pots you can move under shelter is well worth it.

Choose blight-resistant varieties. Plant breeding has come a long way, and several modern varieties shrug off blight that would flatten older types. 'Crimson Crush' is the best-known UK blight-resistant variety and crops well outdoors; 'Mountain Magic' and 'Crimson Plum' are other reliable choices. For an outdoor crop with no greenhouse, a resistant variety is the easiest win of all — there's more on picking one in the guide to choosing tomato varieties.

Space your plants properly. Give each plant room — roughly 45–60cm apart — so air circulates and the leaves dry quickly. Crowded plants stay damp and invite trouble.

Water at the base, not overhead. Make this a habit from day one. Watering into the soil keeps the foliage dry and stops spores splashing about. The same routine helps with several other problems too, including the causes covered in why tomato leaves turn yellow.

Keep tomatoes away from potatoes. Don't grow them side by side, and lift any potatoes promptly if blight appears among them. Practising crop rotation — not growing tomatoes or potatoes in the same patch of ground two years running — also reduces the chance of spores carrying over.

Tidy up at the end of the season. Clear away all old tomato and potato debris in autumn, and never compost blighted material. Spores can overwinter on leftover tubers and plant remains, so a clean-up now means a healthier start next spring.

Blight is one of those problems that feels catastrophic the first time and entirely manageable thereafter. With a resistant variety, a spot under cover and a base-watering habit, most beginners find it stops being a worry at all. For the full picture on raising a healthy crop from seed to harvest, see the main guide to growing tomatoes, and if you're growing potatoes too, the advice on growing potatoes will help you keep both crops blight-free.