Q What is tomato blight?
A Late blight (Phytophthora infestans) is very similar to potato blight. So keep tomatoes well away from potato crops.
It spreads by air-borne spores, so even if you don’t get it one year, it can blow in the next. The spores can only infect a plant when certain conditions are right: the leaves must be damp, the temperature above 10ºC and humidity over 90% for two consecutive days. In other words, weather that's often a feature of the great British summer.
On the stems, the first signs are large, dark-brown spots, which can spread and kill the plant. Similar spots form on the leaves, although they may be lighter or grey in colour. It takes very little time for the leaves to be covered, after which they wither and die. The fruits turn a red/brown marbled colour.
Caption: Tomato blight ruins the crop
Q Can you eat tomatoes if the plant has blight?
A The fruit is not poisonous but blight causes it to be inedible as it doesn't ripen and rots quickly.
Q What can I do to prevent tomato blight?
A Choose outdoor sites that are sheltered from winds. But avoid areas that are so sheltered that air flow is inhibited, leading to damp, still conditions which promote fungus diseases. Avoid growing where potatoes were raised last year – they might be carrying potato blight.
Take care to avoid damage at planting – scars at this time let diseases into the plant when it is at its most vulnerable. Stake plants well, but don’t tie the plants too tightly as the strings may wound the stems. Compost all trimmings and side-shoots. Don’t leave them near the growing plants. If the compost heap is near the tomatoes, cover it with a polythene sheet to stop spores travelling to the plants and infecting them.
Keep the air in greenhouses as dry as you can in humid weather, by ventilating well and avoiding wetting paths, soil and foliage when you water. Opening the door and windows in the morning, when outdoor air is cool and dry, is especially helpful, as long as you don’t suddenly chill the plants.
If you don’t have a greenhouse, try erecting a cover to try to keep the foliage dry at all times. Prevent overheating through adequate ventilation and shading.
Watering the soil without wetting the leaves also helps.
Caption: Opening the greenhouse door and windows will help improve ventilation
Q Are there any tomato varieties that are resistant to tomato blight?
A Yes, there’s been a lot of breeding work to develop resistance to blight. 'Crimson Crush' and 'Mountain Magic' both show good resistance. Alternatively, grow early bush varieties, such as 'Red Alert' and 'Tumbler' which should crop before blight attacks.