Late Blight Management in Tomato and Potato

Late blight (Phytophthora infestans) is arguably the most damaging plant disease of potatoes and tomatoes globally. It was responsible for the Irish Potato Famine (1845–49) and remains a threat to potato and tomato production throughout Southern Africa, particularly in high-altitude, cool, and humid growing regions.

Disease Identification

Late blight spreads with terrifying speed under ideal conditions. The crop can be destroyed within days.

On leaves:

• •Pale green to water-soaked spots, usually starting at leaf margins or tips
• •Spots rapidly expand and turn brown-black
• •White sporulation on the underside of lesions in humid conditions
• •A distinctive musty odour in severely infected crops

On tubers/fruit:

• •Brown, firm, sunken lesions
• •Rust-brown decay extending into the flesh
• •Secondary bacterial soft rot often follows

Conditions for Disease Development

Late blight requires cool temperatures (10–25°C) and high humidity or leaf wetness. The "Beaumont period" rule: if temperature stays above 10°C and relative humidity above 75% for two consecutive 12-hour periods, spray should be applied immediately.

Chemical Control

Spray Programme

1. •Begin spraying before disease onset when weather is conducive
2. •Use a multi-site protectant (mancozeb, chlorothalonil) as the programme backbone
3. •Rotate systemic products from different FRAC groups
4. •Apply on 5–7 day intervals during disease pressure; 10–14 days in dry conditions
5. •Never use metalaxyl-M alone — metalaxyl resistance is widespread; always combine with mancozeb

Resistance Management

FRAC Group 4 (phenylamides: metalaxyl) resistance is common. FRAC Groups 40 and 43 represent newer modes of action with lower resistance risk. Always combine with a multi-site contact fungicide.