Pests & diseases › Late Blight
Late Blight
Devastating fungal-like disease (Phytophthora infestans) of tomatoes and potatoes. Brown-black blotches on leaves, white fluffy mould below in damp weather.
Symptoms
- Dark brown patches on leaves, often with yellow halo
- White fluffy growth on leaf undersides in humid weather
- Brown-black lesions running up stems
- Tomato fruit turning brown and rotting on the truss
- Potato tubers with sunken patches that rot in storage
Lifecycle
Spores arrive on warm humid air June onwards. A 'Smith Period' (two consecutive days above 10°C with 6+ hours of 90%+ humidity) triggers outbreaks. Once you see it, you cannot cure it.
What to do
- Remove and burn affected leaves immediately — do not compost
- On potatoes: cut off all top growth at the first sign, leave tubers in ground 2 weeks then lift
- On tomatoes: pick all green fruit and ripen indoors before disease reaches them
- There is no organic cure — only damage limitation
If it's bad
- Copper-based fungicides as a preventive spray (organic-approved but use sparingly — copper builds up in soil)
- Mancozeb-based products are no longer available to UK home gardeners
Stop it coming back
- Grow blight-resistant varieties: Sarpo Mira/Axona potatoes, Crimson Crush/Mountain Magic tomatoes
- Grow tomatoes under cover — outdoor crops far more vulnerable
- Wide spacing, good airflow, water at base of plant only
- Sign up to free Blightwatch alerts (blightwatch.co.uk) for your postcode
- Three-year rotation of potatoes
Crops it goes for
Plan the plot, track every sowing, cook the harvest
Sowing reminders and harvest forecasts for your own frost dates, the Seed Tin, Scan Centre for any label or packet, and the Potty Mouth Garden Club. Free to start.