Late blight
The potato/tomato disease that can destroy a whole crop in days of cool, wet weather — water-soaked lesions with a white downy mould underneath.
- Pathogen
- Oomycete (P. infestans)
- Spreads by
- Wind & rain-splashed spores
- Favoured by
- Cool, wet, cloudy, humid weather
- Main risk
- Whole-crop collapse in days
Overview
Late blight (Phytophthora infestans) is the most destructive disease of potato and tomato — famous as the cause of the Irish potato famine — and it is feared for its speed. In cool, wet, cloudy, humid weather it can turn a healthy green crop into a rotting, collapsed field within a few days.
It appears as dark green to brown, water-soaked patches, usually starting at the leaf tips and margins, which quickly enlarge and blacken; on the underside, at the edge of a lesion, a fine white downy mould appears in humid conditions (especially in the morning), and the whole plant can rot with a distinctive foul smell. It also blackens stems, and rots potato tubers (firm reddish-brown rot) and tomato fruit. Because it explodes so fast in favourable weather, late blight is managed by watching the weather and forecasts, growing resistant varieties, and spraying protectively BEFORE the disease arrives.
How to identify it
- Dark green to brown, water-soaked patches on the leaves, usually starting at the tips and margins, enlarging fast and blackening.
- A fine white downy mould at the edge of the lesions on the leaf underside in humid weather (especially early morning).
- Blackened stems and petioles, and rapid rotting collapse of the whole plant, often with a foul smell.
- Firm, reddish-brown rot in potato tubers and dark, greasy, firm rot on tomato fruit.
What causes it and when
- The oomycete Phytophthora infestans, whose spores spread by wind and rain-splash and infect through leaf wetness.
- Favoured by cool, wet, cloudy, humid weather with prolonged leaf wetness — it explodes in exactly these conditions.
- Carried over in infected potato tubers (including cull piles and volunteers) and in nearby infected crops.
- Spreads extremely fast once it starts — a field can be destroyed within days in favourable weather.
Cultural & organic control
- Grow resistant/tolerant varieties, plant disease-free (certified) seed tubers, and destroy cull piles and volunteer potatoes that carry it over.
- Watch the weather and any late-blight forecast/advisory — cool, wet, cloudy spells are the danger, and this is when to act pre-emptively.
- Earth up potatoes well so spores cannot reach the tubers, ensure good drainage, and avoid overhead irrigation.
- Rogue out and destroy the first infected plants, and dehaulm (cut and remove tops) before harvest in a blight year so spores do not reach the tubers.
Chemical control
- Spray PROTECTIVELY before the disease appears, timed to cool, wet, cloudy weather — waiting until you see it is often too late.
- Use a protectant fungicide (e.g. mancozeb) in fair weather and switch to CIB&RC-registered systemic/anti-oomycete fungicides (e.g. metalaxyl-based, cymoxanil, etc.) when blight weather or the disease is present; rotate to avoid resistance.
- Cover the crop thoroughly and repeat on the recommended schedule during a blight spell; observe the pre-harvest interval.
- Confirm the current spray schedule and the forecast-based advisory for your area with your local KVK/potato research station.
Prevent it coming back
- Use resistant varieties and certified seed tubers, and eliminate cull piles and volunteers — the main carry-over sources.
- Follow forecast-based, protective spraying and act on blight-favourable weather rather than after symptoms.
- Earth up well, dehaulm before harvest in a blight year, and store only sound tubers.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My potato/tomato crop is rotting fast with dark water-soaked patches. What is it?
That fast-spreading, water-soaked dark blackening — with a fine white downy mould on the leaf underside in humid weather — is late blight, the most destructive potato/tomato disease. It can collapse a whole crop within days of cool, wet weather. It is largely too late to cure once it takes hold, so the answer is protective spraying before it arrives, resistant varieties, and destroying infected plants and tubers.
How is late blight different from early blight?
Late blight is far more aggressive: water-soaked dark lesions with white downy mould underneath that spread rapidly in cool, wet, cloudy weather and can destroy the whole crop in days. Early blight makes slow-spreading concentric-ring "target" spots on the older leaves in warm, humid weather. Late blight needs forecast-based, protective spraying with anti-oomycete fungicides; early blight is less urgent.
Why spray before I even see late blight?
Because late blight explodes so fast in cool, wet, cloudy weather that by the time you see it, it may already be too late to stop a serious loss. The proven approach is protective spraying timed to blight-favourable weather and forecasts — a protectant fungicide in fair weather, switching to systemic anti-oomycete fungicides when blight weather arrives — combined with resistant varieties and clean seed.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.