Late blight (Phytophthora infestans) is one of the most dangerous plant diseases in the organic garden. 🍅 It can blacken tomato and potato leaves overnight. According to ÖMKi researchers: the only effective defence against late blight is prevention.
🌡️ Infection conditions
| Condition | Value |
|---|---|
| 🌡️ Temperature | 10–25°C (optimum 18–22°C) |
| 💧 Wet leaves | At least 4 hours |
| 🌧️ Trigger | Rain, fog, overhead irrigation |
⚠️ Especially dangerous: warm day → cool night with rain or fog. First symptoms can appear within 24–72 hours.
🔬 Symptoms
- 🟡 Oily-looking, yellowish-grey spots on leaves
- 🟤 Spots rapidly turn brown, grow into large irregular patches
- 🌫️ White fuzzy growth on leaf underside (in humid conditions)
- 💀 Blackening, wilting, leaf drop
On fruit: marbled, brown, hard spots from the stem end — rots from inside. On stems: brown sunken patches — shoot dies above.
🛡️ Prevention
- 💧 Drip irrigation — never overhead! Never water in the evening!
- 🌱 Resistant varieties (Phantasia F1, Legend, Ferline)
- ✂️ Regular side-shoot removal + staking → good airflow
- 🌾 Mulching (10–15 cm straw) → no soil splash onto leaves
- 🏠 Polytunnel → rain never touches leaves (ventilate well!)
🧪 Organic treatment
✅ Copper hydroxide / Bordeaux mixture — max. 4 kg metallic copper/ha/year (EU Organic Reg.). Apply preventively before rain! Contact action — does not cure already infected tissue. Remove infected material immediately — do not compost!
🔗 Related articles
📱 BioGarden365 app
- 📷 Photograph leaves → AI identifies and recommends organic treatment
- 🌧️ Weather alert for late blight risk conditions (warm + wet)
- 📖 Garden diary: track copper applications
✨ Summary
Late blight is manageable with consistent prevention: drip irrigation, resistant varieties, good airflow, mulching and preventive copper spraying before risky weather. 🍅
- What is late blight and why is it so dangerous?
Late blight (Phytophthora infestans) is one of the most destructive diseases of tomatoes and potatoes. It can blacken large patches of leaves and fruit overnight, and destroy the entire crop at harvest. This oomycota pathogen caused the Irish Famine (1845–49) in the 19th century.
- What weather conditions favour late blight infection?
Late blight requires 10–25°C temperature (optimum 18–22°C), wet leaves (at least 4 hours), and high humidity. Particularly dangerous: warm day followed by cool night with rain or fog. First symptoms can appear within 24–72 hours after infection.
- How do you recognize late blight on tomato plants?
Leaves show oily-looking, yellowish-grey spots that rapidly turn brown and grow into large irregular patches. In humid conditions, white fuzzy fungal growth appears on the leaf underside. Fruit develops marbled, brown, hard spots from the stem end and rots from inside. Stems show brown sunken patches with wilting above.
- What organic methods prevent late blight?
Drip irrigation (never overhead, never evening watering), resistant varieties (Phantasia F1, Legend, Ferline), good airflow, mulching with 10–15 cm straw, and preventive copper hydroxide spraying before risky weather. A polytunnel keeps rain off leaves (with good ventilation). Remove infected material immediately — do not compost.