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. Following recent research by the James Hutton Institute, blight alerts for potato and tomato growers are now being issued under the Hutton Criteria, which has replaced the Smith Period.
The minimum temperature needed by the disease, 10°C over two consecutive days, is unchanged, but it's now known that relative humidity only needs to be above 90% for six hours on both consecutive days for the disease to spread, rather than 11 hours, as previously thought.
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 Can I compost plants that have had blight?
A Blighted plants can be composted provided the temperature in the compost is high enough, such as in a Hotbin. If you're using an open compost bin it's unlikely to get hot enough to destroy the blight spores, so it's best to burn affected plants or put them in the council green-waste bin as their composting facilities run at higher temperatures. There is a possibility that blight spores could overwinter on other plant material but this hasn't been tested scientifically yet.
Q Can I reuse compost and growing bags that plants with tomato blight were grown in?
A Yes, you can. As with any compost that you're planning to reuse, remove any many of the old roots as possible and carefully search for the c-shaped grubs of vine weevil. When you're ready to plant in the compost, add controlled-release fertiliser to replace the depleted nutrients.
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. Try our Best Buy outdoor tomatoes.