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Late blight
Phytophthora infestans
Fungal diseaseHigh impact
A fast-moving water mould that can devastate tomato and potato crops within days under warm, humid conditions. One of the most serious disease risks in the kitchen garden.
Currently in peak season - extra vigilance worthwhile.
Key facts
- Type
- Fungal disease
- Active season
- Jun - Oct
- Peak season
- Jul - Sep
- Vulnerable stages
- Growing · Harvesting
- Spread rate
- fast
- If confirmed
- Act now
Signs & symptoms
- -dark brown or olive patches on leaves, often with a pale halo
- -white downy growth on the underside of lesions in humid conditions
- -stems developing dark streaks or lesions
- -rapid collapse of foliage in wet weather
- -on tomato: dark sunken patches on fruit
- -on potato: tubers developing reddish-brown rot below skin
Management
Do now
- -Remove and bin (not compost) all affected foliage immediately
- -Avoid wetting foliage when watering - switch to base watering only
- -If infection is severe, consider removing the whole plant to protect neighbours
- -On potato, cut haulm to ground level to reduce risk of tubers being infected
Prevention
- -Choose blight-tolerant varieties where available (e.g. Sarpo Mira for potato)
- -Rotate tomatoes and potatoes to fresh ground each year
- -Space plants at recommended distances to allow airflow
- -Avoid overhead irrigation, especially in the evening
Further reading
- -Spreads most rapidly in 'Hutton period' conditions: two consecutive days of ≥90% humidity and min temp ≥10°C
- -Can travel rapidly via airborne spores - once in the area, all unprotected crops are at risk
- -There is no curative treatment available to home growers - prevention and rapid removal are the only tools