Tomato Late Blight: How to Identify, Treat & Prevent Phytophthora infestans (2026)
Quick Answer
Apply copper-based fungicide (Bordeaux mixture at 10g copper sulfate + 10g lime per liter) every 7 days starting at the first sign of water-soaked lesions. Remove and burn all affected tissue immediately — do NOT compost it. If the disease has spread beyond 30% of foliage, apply a systemic fungicide containing Metalaxyl + Mancozeb (Ridomil Gold MZ) at 2.5g/L. Late blight moves fast — a 48-hour delay can mean total crop loss.
What Does It Look Like?
🍅
Early stage
Irregular, water-soaked, dark green to brown lesions appear on leaf edges and tips — often starting on the upper canopy where dew accumulates. Lesions look greasy or oily when wet. A faint white fuzzy growth (sporangia) may be visible on the underside of leaves in humid mornings.
🍅
Advanced
Lesions expand rapidly into large brown-black blotches covering entire leaflets. White sporulation is clearly visible on leaf undersides. Brown streaks appear on stems. Fruit develops firm, dark brown, leathery patches starting from the stem end. A distinctive acrid smell may be noticeable in the field.
🍅
Severe
Entire plants collapse within 3–5 days — leaves turn black, stems rot, and fruit becomes unmarketable. The field has a characteristic rotting odor. Under high disease pressure, 100% crop loss occurs if untreated. Even partially affected fruit will rot in storage.
How to Treat It
Apply Bordeaux mixture (copper sulfate + hydrated lime)
Mix 10g copper sulfate + 10g hydrated lime in 1 liter of water. Stir the copper sulfate into water first, then add the lime solution. Spray to full coverage on all foliage, stems, and fruit — both surfaces of leaves. The alkaline copper creates a protective barrier that kills spores on contact.
Timing: Every 7 days preventively; every 5 days during active outbreaks
Spray baking soda solution as a supplementary treatment
Dissolve 1 tablespoon baking soda (sodium bicarbonate) + 1 teaspoon liquid soap + 1 tablespoon vegetable oil in 1 liter of water. The alkaline pH inhibits spore germination. Less effective than copper but useful between copper applications.
Timing: Every 5–7 days, alternate with copper sprays
Remove and destroy all affected tissue immediately
Cut affected branches 15cm below the visible lesion boundary. Place directly into a plastic bag — do not drop pieces on the ground. Burn the material or seal it in bags for municipal waste. NEVER compost late blight-infected tissue as spores survive composting.
Timing: As soon as symptoms are spotted — every inspection
Apply Trichoderma harzianum as a biocontrol agent
Mix 5g of Trichoderma powder per liter of water and apply as a soil drench and foliar spray. This beneficial fungus competes with Phytophthora and can reduce infection severity by 30–50% when used preventively.
Timing: Every 10–14 days as a preventive measure
Best for: small farms, organic certification, home gardens
Metalaxyl + Mancozeb (Ridomil Gold MZ 68% WP)
Chlorothalonil (Daconil 75% WP)
Cymoxanil + Mancozeb (Curzate M8)
Best for: large-scale farming, severe outbreaks
Plant resistant or tolerant varieties
Choose varieties with Ph-2 or Ph-3 resistance genes: 'Mountain Magic', 'Defiant PhR', 'Iron Lady', 'Plum Regal'. In East Africa, look for 'Tengeru 97' or 'Tanya'. No variety is fully immune — resistance slows the disease but doesn't eliminate it.
Maintain 90–120cm spacing between plants
Wide spacing allows rapid leaf drying after rain or dew. Late blight requires 8+ hours of leaf wetness for infection — anything that reduces drying time is critical. Prune lower branches to improve ground-level airflow.
Avoid overhead irrigation entirely
Use drip irrigation or furrow irrigation. Overhead sprinklers create exactly the prolonged leaf wetness that Phytophthora needs. If you must use sprinklers, irrigate early morning so leaves dry by midday.
Scout fields every 2–3 days during the rainy season
Walk your field in a W-pattern, checking upper and lower leaf surfaces. Focus on field edges (closest to wind-blown spore sources) and low-lying wet areas. Early detection gives you a 48-hour window before the disease explodes.
The best treatment is prevention
When This Problem Occurs
🌡️
Temperature
10–25°C (optimal 15–20°C — this is a cool-weather disease)
💧
Humidity
Above 90% relative humidity, with prolonged leaf wetness (8+ hours)
🌧️
Rainfall
Frequent rain, drizzle, or heavy dew creates ideal infection conditions
📅
Season
Late rainy season, cool highland areas, and transitional seasons
For Tomato Farmers
For tomato farmers in the tropics: late blight is your #1 threat during the cool rainy season. In highland areas of East Africa (1,500–2,500m elevation), expect late blight pressure from March–May and October–December. Start preventive copper sprays 2 weeks before the rains begin — waiting for symptoms means you've already lost the first battle. Keep a stock of Ridomil Gold MZ for emergency curative treatment. If you lose more than 50% of foliage, it's often more economical to destroy the crop and replant than to continue treatment. Always stake and prune — fruit touching the ground will rot first.
Farmers Also Ask
Can tomato plants recover from late blight?
Partially. If caught early (less than 20% of foliage affected), aggressive treatment with systemic fungicides like Metalaxyl + Mancozeb combined with removal of all infected tissue can save the plant. However, severely infected plants (50%+ foliage affected) rarely produce a worthwhile harvest and should be removed to protect neighboring plants.
Is late blight the same disease as early blight?
No — they are completely different organisms. Early blight (Alternaria solani) is a true fungus that causes concentric ring spots (target spots) and progresses slowly from the bottom up. Late blight (Phytophthora infestans) is an oomycete that causes rapid, water-soaked lesions with white sporulation and can destroy a field in days. Late blight is far more destructive.
Can I eat tomatoes from a late blight-infected plant?
Fruit without visible lesions is safe to eat. However, infected fruit (with brown, leathery patches) should not be consumed as secondary bacterial infections often develop in the damaged tissue. Even seemingly healthy fruit from severely infected plants will have a shorter shelf life — use it within 2–3 days of harvest.
How long do late blight spores survive in soil?
Phytophthora infestans does NOT survive well in soil without living host tissue — unlike many other pathogens. Spores typically survive only 3–4 weeks in soil. The primary survival mechanism is on infected potato tubers left in the ground, volunteer potato/tomato plants, and on infected transplants. This is why cleaning up crop debris is critical.
Get the Complete Farming Toolkit
AI disease scanner • Pest identification • Weather alerts • AI chat expert • Works offline
3 free scans per day • 10 languages • No internet needed