Tomato blight, a fungal infection called Phytophthora infestans, spreads by wind and water-splash. It attacks tomatoes and potatoes and is triggered by warm, wet conditions, making outdoor tomatoes more susceptible than those in a greenhouse. The crop is quickly ruined and even if you pick the tomatoes at the first sign of infection, you can’t stop them rotting.
How to identify tomato blight
Spread by airborne spores that can be carried over 30 miles on the wind, tomato blight is most prevalent when conditions are warm and wet. Outdoor tomatoes are more susceptible to blight than those growing in a greenhouse.
When affected by blight, ripening tomatoes develop brown sunken spots, which spread to the leaves and stems.
In this video, Monty Don explains what to do if blight affects your tomatoes, and explains whether you can still eat the tomatoes growing on blight-infected plants:
How to prevent tomato blight
Keep tomatoes dry
Grow tomatoes in a greenhouse or polytunnel if possible, as this will keep the leaves dry and help fruits ripen sooner than those grown outside.
Never plant tomatoes in soil or compost that has previously contained diseased plants.
Support tomato plants with a stake, including bush varieties, to keep their leaves off the soil.
If growing tomatoes outside and blight hits, try placing an umbrella of polythene or a plastic roof over tomatoes to keep the rain off them. This will stop rain splashing onto the leaves and reduce the likelihood of infection.
When watering tomatoes, water in the morning, so the plants don’t sit damp all night. Take care to water only the soil or compost, keeping the leaves dry. Remember, tomatoes taste better if the plants aren’t overwatered.
Feed tomatoes with fertilisers that are high in potassium, such as dedicated tomato feeds. Never feed tomatoes with a high-nitrogen fertiliser as this boosts leaf production, making blight more likely.
Provide good ventilation
Pick a sunny, well-ventilated spot for growing tomatoes outdoors. Leave enough space between plants for air to circulate – don’t be tempted to plant closer than the seed pack recommends.
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.
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.
Avoid planting tomatoes near potatoes
Don’t plant tomatoes near potato crops, as these are also susceptible to blight and their proximity will make it easier for the blight to spread between crops.
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.
Grow early tomatoes
Cherry tomatoes tend to be less likely to catch blight than beefsteaks, because they ripen earlier and are often harvested before blight hits.
Practice good hygiene
Once blight hits, destroy blighted foliage to reduce chances of further infection. Don’t leave potato tubers in the ground at harvest as they could harbour blight. Blighted foliage can be composted away from the crop as spores need a living host to survive for more than a few weeks.
Blight resistant tomatoes to grow
Some tomatoes have been bred with a degree of blight tolerance – they may still catch the disease but are able to survive and yield some healthy fruit. Here, Monty Don picks the fruit of blight-resistant tomato variety ‘Losetto’:
We’ve picked our favourite blight-resistant tomatoes to grow, below.
This unusual cordon tomato produces strawberry-shaped, cherry tomatoes that have a super-sweet flavour. It’s early to ripen, and shows some resistance to blight.
A blight-tolerant cordon tomato, reliable, and resistant to fusarium and verticillium wilt. It produces a heavy crop of medium-sized, deep red fruit with an excellent flavour.
Tomato ‘Fantasio’ F1
A superbly flavoured cordon tomato, bearing a prolific crop of medium-sized fruit. Blight tolerant and resistant to fusarium and verticillium wilt, it’s best grown under glass.
Tomato ‘Ferline’ F1
This cordon variety has some blight tolerance, and resistance to fusarium and verticillium wilt. ‘Ferline’ bears heavy crops of tasty, medium-sized fruit.
Bush variety ‘Latah’ produces very early, large, cherry tomatoes, with excellent flavour. Its sprawling habit and well-spaced leaves help fruits ripen before blight strikes.
Bush variety ‘Legend’ holds the RHS Award of Garden Merit and has some blight tolerance. A beefsteak variety with heavy crops of large, it produces almost seedless fruit with superb flavour.
Tomato ‘Lizzano’ F1
A vigorous variety with lots of sweet, round cherry fruits and a trailing habit, ‘Lizzano’ is perfect for pots and hanging baskets. It also has good blight tolerance.
Tomato ‘Losetto’ F1
This very blight-tolerant variety produces masses of bright red, sweet cherry tomatoes. ‘Losetto’ is ideal for growing in pots.
Tomato ‘Red Alert’
Bush tomato ‘Red Alert’ bears a lot of small, sweet, cherry tomatoes. Its sparse leaves reduce humidity and fruits ripen very early, so are often harvested before blight strikes.