Blight is a disaster for tomatoes, but there are ways to avoid it to ensure a healthy crop. Tomato blight thrives in warm, humid conditions, so our wet summers are the perfect environment. When there’s blight about, you can’t completely protect tomato plants, but you can decrease the risk of them catching it if you act early.
Begin by thoroughly cleaning your greenhouse or polytunnel to ensure it’s free from overwintering spores. Outdoors, dig through the soil to check there are no potatoes left in the ground as these may harbour blight that can spread to the next potato and tomato crops.
Pick a sunny and well-ventilated spot for growing tomatoes outdoors in summer.
Grow tomatoes in a greenhouse or polytunnel if possible; it will keep leaves dry and help fruits ripen early.
A beautiful large beef tomato ripening in sunshine on the vine
- Don’t plant in soil or compost that has previously contained diseased plants
- Erect a plastic or polythene canopy above outside plants if possible, to keep rain off like an umbrella. Ensure there’s enough ventilation to prevent condensation
- Leave enough space between plants for air to circulate – don’t be tempted to plant closer than the seed pack recommends
Water in the morning, so that plants don’t sit damp all night.
Take care to only water the soil or compost, keeping the leaves dry. Remember, tomatoes taste better if the plants aren’t overwatered.
Watering a tomato plant in a pot with a watering can
- Cover the soil with a permeable membrane to help keep leaves dry
- Support plants, including bush varieties, to keep leaves off the soil
- Remove lower leaves, and some higher up, to increase air circulation. Vigorous bush varieties produce too many leaves and benefit from drastic pruning to thin the foliage and encourage good ventilation
- Feed only with fertilisers that are high in potassium, such as dedicated tomato feeds. Never feed tomatoes with a high-nitrogen fertiliser; it boosts leaf production, making blight more likely
- Stop greenhouses or polytunnels becoming too humid. Keep them well-ventilated, mop up water and, as the evenings turn cooler, use a heater to reduce condensation
- There are copper-based, chemical sprays available which offer some protection against blight. However, these must be sprayed regularly throughout the growing season as a preventative, and have no effect once a plant is infected
Check plants regularly for blight, from summer onwards, and dispose safely of badly diseased plants.
When the weather conditions are optimum for blight, remember to check your tomatoes and potatoes more frequently.
Ripe tomatoes on the vine with rings of scarring due to skin cracking