The short answer
Keep the leaves dry, give them air, water at the base in the morning, and grow them under cover if you can. Blight arrives in warm wet weather and spreads on wet foliage.
- Water at the base, never over the leaves, and in the morning so any splash dries.
- Space them properly so air moves between plants.
- Remove lower leaves as the plant grows, so nothing touches the soil.
- Pinch out side shoots on cordon varieties, which improves airflow as well as cropping.
- Under cover is much safer. A greenhouse or even a rain shelter dramatically reduces the risk.
- Choose blight-resistant varieties if you grow outdoors.
What will ruin it
If blight does take hold, remove and bin the affected plants. Do not compost them and do not grow tomatoes or potatoes in the same spot next year.
Why it works
Blight spores need several hours of leaf wetness at warm temperatures to infect. Every part of the method is about denying them that window.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.