Preventing Late Blight in Tomato and Potato: A Season-Long Playbook

Published 2026-01-22 · Updated 2026-04-10 · For Tomato and potato growers, students

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.