Pests & diseases › Late Blight
Late Blight
Devastating oomycete disease (Phytophthora infestans) of tomatoes and potatoes. Brown-black blotches on leaves, white fuzz below in humid weather. The same pathogen that caused the Irish Potato Famine.
Symptoms
- Dark brown patches on leaves, often with a yellow halo
- White fluffy growth on leaf undersides in humid weather
- Brown-black lesions on stems
- Tomato fruit turning brown and rotting on the vine
- Potato tubers with sunken patches that rot in storage
Lifecycle
Spores travel on humid air June–September. Triggered by cool nights and warm humid days. Once present, cannot be cured.
What to do
- Remove and bag affected leaves immediately — do not compost
- Potatoes: cut all top growth at first sign, leave tubers 2 weeks then dig
- Tomatoes: pick green fruit and ripen indoors
- No organic cure — only damage limitation
If it's bad
- Copper-based fungicides as preventive (organic-approved but use sparingly)
- Chlorothalonil — homeowner products being phased out, check current state regulations
Stop it coming back
- Resistant varieties: Mountain Magic, Defiant PhR, Iron Lady tomatoes; Elba, Defender, Kennebec potatoes
- Wide spacing, good airflow, water at base of plant only
- Sign up for USAblight.org regional alerts
- Three-year rotation of potatoes and tomatoes
Crops it goes for
Plan the plot, track every sowing, cook the harvest
Sowing reminders and harvest forecasts for your own frost dates, the Seed Tin, Scan Centre for any label or packet, and the Potty Mouth Garden Club. Free to start.