Late Blight
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Remove affected material now
Take out the affected leaves, shoots or whole plants before the next wet spell, bag them as you cut, and wash your hands and tools before touching healthy plants.
Wind-blown spores
Spores travel on air well beyond the affected bed, so a neighbouring garden can be the source. Spacing and airflow matter more than distance from your own infected plant.
It carries over to next season in soil, debris or seed — which is why rotation matters here, not just treatment.
Keep out of the compost
This survives a garden heap. Bag it for municipal green waste, which is composted industrially at temperatures a garden heap cannot reach, or burn it where that is permitted.
When and where to look
Scout the planting while temperatures hold between 10°C and 25°C, with humidity above 90%, after rain or heavy dew, while the leaves are still wet. Risk builds after roughly 10 unbroken hours of those conditions, so the morning after such a night is the moment to look.
Rule these out first
Same category, and at least one host crop in common — so they are genuinely confusable in the same part of the garden.
The signs that lead here
Full problem lists for these crops
About
Late blight (Phytophthora infestans) is the most destructive disease of tomato and potato, capable of wiping out a crop in days during cool, wet weather. It caused the Irish potato famine and remains a serious threat for home and commercial growers alike.
Symptoms
Dark, water-soaked greasy patches on leaves that rapidly turn brown; white fuzzy sporulation on leaf undersides in humid mornings; firm brown lesions on stems and fruit. Whole plants can collapse within days.
Organic Treatment
Remove and bag infected foliage immediately. Apply copper-based fungicide preventively before symptoms in wet spells. Improve airflow and avoid overhead watering. Severely infected plants should be destroyed, not composted.
Chemical Treatment
Protectant fungicides (chlorothalonil, mancozeb) on a 5–7 day schedule, or systemic products (cymoxanil, mandipropamid) when pressure is high. Rotate modes of action to avoid resistance.
Prevention
Plant resistant varieties, space widely for airflow, water at the base in the morning, stake plants off the ground, and never save tubers or fruit from infected plants.
Crops at risk
Related problems
Late Blight risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.