How do I prevent tomato and potato blight?

Late blight is a fungus-like organism that arrives on the wind during warm humid spells. It can destroy a crop in under a week. Everything useful is preventative — resistance, airflow, and keeping foliage dry.

What you'll need

• Blight-resistant varieties
• Greenhouse or cover (optional)
• Secateurs (optional)

Step by step

1. Learn what to look for.Brown patches on leaves with a pale yellow halo, dark streaks on stems, and brown sunken patches on fruit. It spreads downwards and outwards fast.
2. Choose blight-resistant varieties.Several tomato and potato varieties now carry good resistance. In a wet climate this is the single most effective decision you make.
3. Grow tomatoes under cover if you can.A greenhouse or a polytunnel keeps the foliage dry, and dry foliage largely prevents infection. Outdoor tomatoes in a wet summer are at real risk.
4. Water at the base, never over the leaves.Wet foliage is what blight needs to establish. Water in the morning so any splashes dry quickly.
5. Space plants properly and remove lower leaves.Airflow dries foliage. Crowded plants stay humid at their base, which is exactly where infection starts.
6. Watch for blight warnings during warm humid spells.Several services forecast the conditions that trigger it. It gives a few days' notice to act.
7. Remove and destroy infected material immediately.Do not compost it. Bag it and remove it from the site — home compost heaps do not get hot enough to kill the spores.
8. Cut potato foliage off at the first sign.Remove the haulm to ground level. It stops spores washing down to the tubers, and the potatoes underneath usually keep.

Tips

• Never plant tomatoes and potatoes near each other. They share the disease and one infects the other readily.
• Rotate where you grow potatoes each year. Spores overwinter in the soil and in any tubers left in the ground.
• Earlier potato varieties often finish before blight season starts, which sidesteps the problem entirely in wetter regions.

Common mistakes

• Composting infected plants — Home compost rarely reaches a temperature that kills the spores, so you reintroduce it next year.
• Watering tomatoes overhead — Wet foliage is the condition blight requires. Watering at the base removes the main route of infection.
• Waiting to see if the plant recovers — It will not, and it infects everything nearby while you wait. Act at the first sign.

Questions people ask