Blight

What blight actually is

Blight — properly late blight — is caused by a fungus-like organism called Phytophthora infestans. It is the same disease that triggered the Irish potato famine, and it remains the single biggest threat to home-grown potatoes and tomatoes in the UK.

Crucially, potato blight and tomato blight are the same organism. Both crops are members of the same family (the nightshades), so spores blowing off an infected potato patch can easily land on your tomatoes, and vice versa. If you grow both, treat them as one problem.

When it strikes

Blight is a warm-and-wet disease. It needs a stretch of mild, humid, damp weather to take hold — roughly when temperatures sit around 10°C or higher with high humidity for a day or two. In a typical UK season that means mid-summer to early autumn, usually from late June or July onwards, and it spreads alarmingly fast once conditions are right.

You may hear gardeners mention "Smith periods" or "blight alerts" — these are warnings issued when the weather has been muggy enough for an outbreak. They are a useful nudge to inspect your plants and act early.

The signs are unmistakable once you know them: irregular brown or grey-brown patches on leaves that spread within days, dark streaks on the stems, and rotting fruit or tubers. In wet weather a fuzzy white mould often appears on the underside of affected leaves.

Why prevention beats cure

There is no home cure once blight has properly taken hold — by the time you spot it, the spores have usually already spread. So the whole game is prevention and slowing it down:

• Choose blight-resistant varieties. Potatoes like 'Sarpo Mira' and tomatoes like 'Crimson Crush' shrug off most outbreaks. These are widely sold by UK suppliers such as Thompson & Morgan and Suttons.
• Grow tomatoes under cover. A greenhouse or polytunnel keeps the leaves dry, and dry leaves rarely catch blight. Outdoor tomatoes are far more vulnerable.
• Space plants and water at the base so air moves freely and foliage dries quickly.
• Lift early potatoes before peak blight season — first earlies are often harvested before the worst risk arrives, while maincrop potatoes sit in the ground longest and are most exposed.

If blight does appear on potatoes, cut off and bin (don't compost) all the top growth straight away; the tubers below can often still be saved if you lift them after a couple of weeks. On tomatoes, sadly, an outbreak usually means the end of the plant.

Practising sensible crop rotation and never composting infected material both help break the cycle for next year. For a full walkthrough, see our guide to tomato blight.