Late blight (Phytophthora infestans) is one of the most devastating diseases for UK vegetable growers. It can destroy a potato or tomato crop within days in warm, wet conditions. I lost most of an outdoor tomato crop to it in a wet August a few years back, which is the only reason I now grow tomatoes under cover wherever I can. Early identification and swift action are essential.
Founder & Head Grower, SoilCommander · RHS Level 2 · 12+ years growing in Yorkshire
I've been growing vegetables on my Yorkshire allotment and raised beds since 2012. Everything I write is based on what I've actually grown, failed at, and eventually got right in a real UK climate.
What Is Late Blight?
Late blight is a water mould (oomycete) that spreads rapidly in humid conditions above 10°C. It is not a true fungus but behaves similarly, producing spores that travel on wind and rain. Because it is so water-dependent, RHS guidance is to destroy infected haulms immediately and keep a strict four-year crop rotation. The UK's wet summers make blight a near-annual threat, particularly from July onwards.
Identifying Blight Symptoms
| Plant | Symptom | Where to Look |
|---|---|---|
| Potato | Dark brown patches on leaves with pale yellow halo; white mould on leaf undersides in humid conditions | Lower leaves first, spreading upward |
| Potato | Brown rot spreading through tubers; foul smell | Tubers at harvest or in store |
| Tomato | Dark brown/black patches on leaves and stems; greasy appearance | Leaves, stems, and fruit |
| Tomato | Brown marbling inside fruit; fruit rots rapidly | Green and ripening fruit |
Blight Risk Conditions (Smith Periods)
A Smith Period is when blight risk is high: two consecutive days where minimum temperature is 10°C or above and relative humidity exceeds 90% for at least 11 hours each day. The UK Met Office issues blight warnings — check these during summer.
What to Do If Blight Strikes
- Potatoes: Cut off all foliage immediately and remove from the plot. Leave tubers in the ground for 2–3 weeks — the skin will set and reduce tuber infection risk. Harvest promptly and inspect every tuber before storing.
- Tomatoes: Remove and bin (do not compost) all affected leaves and fruit. If the stem is infected, the plant is unlikely to recover — remove entirely.
- Never compost blighted material — bag and bin it or burn it.
Prevention Strategies
- Grow blight-resistant varieties — potatoes: Sarpo Mira, Sarpo Axona, Setanta; tomatoes: Crimson Crush, Lizzano, Fantasio.
- Rotate crops — never grow potatoes or tomatoes in the same ground two years running.
- Improve airflow — space plants well and stake tomatoes to keep foliage off the ground.
- Water at the base — avoid wetting foliage; use drip irrigation or water cans directed at the root zone.
- Copper-based fungicide — approved for organic use; apply preventatively during high-risk periods. Not a cure once blight is established.
- Grow tomatoes under cover — a polytunnel or greenhouse significantly reduces blight risk by keeping foliage dry.
Plan blight-resistant crop rotations with our UK Vegetable Garden Planner PDF — £19.
Frequently Asked Questions
Can I eat potatoes affected by blight?
Mildly affected tubers with only surface browning can sometimes be used if the rot is cut away and the potato cooked immediately. However, tubers with deep brown rot or a foul smell should be discarded. Never store blighted potatoes as the rot spreads rapidly.
Is blight the same as early blight?
No. Late blight (Phytophthora infestans) is the most serious and fast-moving disease. Early blight (Alternaria solani) causes concentric ring-shaped spots on older leaves and is less destructive. This guide focuses on late blight.
When is blight season in the UK?
Blight risk is highest from July through September, when warm, humid conditions are most likely. However, blight can occur earlier in mild, wet summers. Monitor Met Office blight warnings from June onwards.
Does blight survive in the soil over winter?
Late blight does not survive long in open soil, but it can overwinter in infected potato tubers left in the ground. Always remove all potato material after harvest and never leave volunteer potatoes to grow the following year.
Can I save seeds from blighted tomatoes?
It is not recommended. Blight can infect fruit and potentially contaminate seeds. Use seeds from healthy, disease-free fruit only, or buy certified seed each year to reduce disease risk.
Fix The Cause, Then Plan The Next Crop
Troubleshooting works best when you improve the growing conditions and record what changed for the next season.
Plan the next step
Use the printable UK Vegetable Garden Planner to turn this guide into sowing dates, bed layouts, and weekly garden tasks.