Late blight is the disease that caused the Irish potato famine — and it still destroys entire fields in 10 days when conditions are right. Cure is nearly impossible; prevention is everything. Here is the plan that commercial growers use to keep it out.
Pre-plant: cultivar and site
Choose blight-tolerant cultivars whenever possible — Sarpo Mira and Defender in potatoes; Mountain Magic, Iron Lady and Legend in tomatoes. Never plant on the same bed for at least 3 seasons. Space plants for airflow: 40–50 cm within row, 90 cm between rows.
Water at the base, in the morning
Late blight spores need 6+ hours of leaf wetness at 10–20 °C to germinate. Drip irrigation eliminates most of the risk. If you must sprinkle, do it before 9 a.m. so leaves dry by evening.
Scouting: check the underside of lower leaves twice a week
First lesions appear as water-soaked, greasy patches on lower leaves. A white fungal fringe on the underside in humid mornings confirms it. Once you see it, you have about 5 days before it colonizes the whole plant.
Preventive fungicide schedule
Start 14 days before the historical outbreak date in your region. Rotate chlorothalonil (2 mL/L) with mancozeb (2 g/L) every 7 days as a protectant. Add copper hydroxide during heavy rain events for bacterial cover. Switch to systemic mefenoxam only after an outbreak is confirmed, and never more than twice per season to prevent resistance.
End-of-season cleanup matters as much as spraying
Volunteer potatoes and infected tubers are the main overwintering source. Harvest all tubers, destroy culls, and never leave infected foliage on the field.
Key takeaways
- Prevention beats cure — late blight has no reliable rescue once established.
- Cultivar choice, spacing and drip irrigation remove most of the risk.
- Scout under lower leaves twice a week during cool wet weather.
- Rotate a protectant fungicide program every 7 days; reserve systemics for confirmed outbreaks.
Related in the pathology library
Products you can buy for this problem
Typical registered products for the diseases covered in this article, with their active ingredient, mixing rate, spray interval and pre-harvest interval. Rotate FRAC/IRAC groups to avoid resistance.
Mancozeb 80% WP
Active ingredient: Mancozeb
Copper oxychloride 50% WP
Active ingredient: Copper oxychloride
Ridomil Gold MZ (metalaxyl-M + mancozeb)
Active ingredient: Metalaxyl-M 4% + Mancozeb 64%
Cymoxanil + famoxadone
Active ingredient: Cymoxanil 30% + Famoxadone 22.5%
Doses shown are typical registered field rates. Always read the product label, confirm the product is registered for your crop and country, respect the pre-harvest interval (PHI), and wear the protective equipment the label specifies. PlantWatch AI provides agronomic guidance, not a legal spray recommendation.
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