Late blight is a water mould that destroys potatoes and tomatoes in cool, wet conditions, spreading rapidly enough to wipe out crops in days. Symptoms include water-soaked patches on leaves, stems, and fruits, followed by white mould on leaf undersides. Remove infected parts immediately, improve air circulation, and avoid overhead watering to slow spread.
Late blight is a devastating disease caused by the water mould Phytophthora infestans, primarily affecting potatoes and tomatoes in UK gardens. It thrives in cool, wet conditions, spreading rapidly to destroy foliage, stems, fruits, and tubers. Infamous for the Irish Potato Famine, it can wipe out crops in days if unchecked. You can limit damage with vigilant monitoring, prompt removal of infected parts, and cultural practices that reduce humidity around plants. Early action is key to saving your harvest.
Identification
Look for large, irregular dark brown or black lesions on leaves, often starting on lower ones after wet weather. Lesions have pale green or grey edges and are not confined by leaf veins. Under humid conditions, check leaf undersides for white, fuzzy mould – the pathogen's spores. Stems show firm, dark brown lesions with rounded edges. On tomatoes, fruits develop firm, dark brown spots that turn mushy. Potato tubers have reddish-brown rot extending inwards. To confirm in dry weather, seal a suspect leaf in a damp bag overnight; white sporulation confirms late blight.
Damage & symptoms
Affected leaves develop greasy green-black patches that expand into large brown blotches, causing rapid wilting and defoliation as if frosted. Entire plants collapse within days in wet spells. Tomato fruits get leathery, water-soaked brown spots that rot quickly, often invaded by bacteria. Potato stems brown and die above infection sites; tubers rot internally with discoloured, firm flesh turning soft. High humidity reveals white mould on all parts. Untreated, it leads to total crop loss, with decay continuing post-harvest.
Lifecycle
Phytophthora infestans spreads via airborne spores from infected plants, needing just 12 hours of leaf wetness to infect. Ideal conditions are 15-21°C days and 10-15°C nights with high humidity. The cycle completes in 3-5 days: spores germinate, penetrate tissue, and produce new spores. It survives on volunteer potatoes, cull piles, or infected tubers/debris over winter. New seasons start from contaminated seed tubers, transplants, or windblown spores from afar.
Prevention — Choose resistant varieties
Grow blight-resistant potatoes like Sarpo Mira, Sarpo Axona, or Defender, and tomatoes such as Sarpo Daphne or Mountain Magic. These show good tolerance in UK trials, though no variety is fully immune. Plant certified seed tubers and transplants free from disease to avoid introducing the pathogen.
Prevention — Improve air circulation
Space plants widely – 60cm for potatoes, 45-60cm for tomatoes – and stake cordons to keep foliage off the ground. Avoid overhead watering; use drip irrigation or water soil early in the day. Prune lower leaves on tomatoes for better airflow, reducing humid microclimates where blight thrives.
Prevention — Rotate crops and clear debris
Never plant potatoes or tomatoes where either grew in the past two years. Dig up all potatoes at harvest, removing volunteers promptly. Destroy crop debris by burning or council green waste – never compost, as it rarely kills the pathogen.
Prevention — Monitor weather forecasts
Watch for prolonged wet, mild spells (10-20°C). UK blight forecasting tools like those from AHDB can warn of risk. Act pre-emptively by removing lower leaves or applying protectants when forecasts predict ideal conditions.
Organic control — Remove and destroy infected plants
At first sign of symptoms, cut out affected leaves or whole plants immediately. Bag and bin them or burn if permitted – do not compost. This halts spore spread. Check daily in wet weather, focusing on lower canopy.
Organic control — Apply copper-based sprays
Use Bordeaux mixture or copper oxychloride (approved organic fungicides in the UK) as a protectant before symptoms or at early signs. Spray every 7-10 days in high-risk weather, covering all foliage thoroughly. Stop use two weeks before harvest.
Organic control — Mulch with compost
Lay thick organic mulch around plants after planting to suppress soil splash carrying spores to leaves. This also retains moisture without wetting foliage. Refresh after rain to maintain barrier.
Organic control — Encourage natural predators
Boost garden biodiversity with flowers to attract beneficial insects. Though no direct predators for blight, healthy ecosystems aid resilience. Avoid broad disruptions.
Frequently asked questions
Can I still eat potatoes or tomatoes with late blight?
Cut away healthy-looking tubers or fruits, ensuring at least 1cm clear margin from any lesions. Store in cool, dry conditions and check regularly for rot. Discard if internal browning appears. Tomatoes with surface spots can be skinned and cooked if firm inside, but avoid if mushy.
Does late blight affect indoor tomatoes?
Yes, high humidity in greenhouses mimics outdoor conditions, promoting spread. Ventilate well, space plants, and lower humidity below 85%. Use resistant varieties and monitor transplants closely, as they often introduce it.
How do I know if my seed potatoes are infected?
Inspect for firm, reddish-brown rot under skins or dark lesions. Plant only certified seed from reputable suppliers. If in doubt, chit in isolation and discard any showing symptoms before planting.
Will baking soda or milk sprays control blight?
Folk remedies like bicarbonate sprays may slightly suppress symptoms but lack reliability against aggressive late blight. They work best as part of integrated prevention, not cures. Rely on proven organics like copper for real protection.
How quickly does late blight spread in my garden?
In warm, wet weather, spores can infect new plants overnight, with visible symptoms in 3-5 days. Whole rows collapse in a week. Daily patrols and immediate removal are essential to contain it before wind spreads spores.