Potato and tomato blight

What it is

Blight is caused by a water mould, a fungus-like organism that struck Irish potato crops in the 1840s and is still the most destructive disease of potatoes and outdoor tomatoes in the UK and Ireland. In a wet July or August, a healthy-looking row can be blackened inside a week, and the spores can travel many miles on the wind from other allotments and farms.

Symptoms

Watch for dark brown, water-soaked patches on the leaves, often with a pale edge and, in damp weather, a fine white fringe of spores underneath. Stems develop brown streaks and the whole top can collapse into a soggy, smelly mess. On tomatoes, green and ripening fruit develops firm, bronzed or brown blotches that quickly rot. Infected potato tubers show sunken reddish-brown patches with rusty, granular flesh beneath the skin.

Cause

Spores need warm, humid conditions with wet leaves, typically several days in a row with temperatures above about 10°C and high humidity, which is why outbreaks usually arrive from late June onwards. Rain washes spores from the leaves into the soil, where they reach the tubers. The disease survives the winter in infected tubers left in the ground, in compost heaps and in rubbish heaps of discarded potatoes.

Control

Check plants daily once warm, muggy weather arrives. Remove infected tomato leaves straight away and bag them. On potatoes, cut the foliage down to soil level as soon as blight takes hold, then wait about two weeks before lifting so spores on the surface die off and the skins harden. Eat or discard damaged tubers rather than storing them. Protective fungicides exist but must go on before infection; for most gardeners they are a last resort, used strictly to the label.

Prevention

Grow first or second early potatoes, which are often harvested before blight arrives, or try strongly resistant varieties such as the Sarpo range. For tomatoes, a greenhouse or polytunnel keeps leaves dry, and outdoor types bred for resistance, such as Crimson Crush, cope far better. Dig out every stray potato, practise crop rotation and water the soil rather than the foliage.

Plants often affected

• Potatoes
• Tomatoes

Could it be something else?

These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.

• Chocolate spot
• Grey mould
• Allium leaf miner
• Blossom end rot

Months to keep an eye out: June, July, August and September.

Sources

Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.

• Wikipedia — Phytophthora infestans
• University of Minnesota Extension — Late blight