Tomato blight

Tomato blight, a fungus-like infection called Phytophthora infestans, spreads by wind and water-splash. It attacks tomatoes and potatoes and is triggered by 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, mostly because they are exposed to rainfall on their leaves, whereas greenhouse plants can be watered right into the roots. Greenhouse crops are also slightly less likely to be exposed to the air-borne spores from other plants nearby.

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 the warmer environment will help fruits ripen sooner than those grown outside.

Use short stakes with bush types to keep their leaves away from the soil.

Take care to water only the soil or compost, keeping the leaves dry.

Provide good ventilation

Pick a sunny, sheltered but 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.

Perform regular sideshooting on cordon-type tomatoes, or they will grow bushy and be more susceptible to blight.

On cordon types, remove lower leaves where fruits have already formed, to increase air circulation, let in light and speed up ripening. Vigorous bush varieties will tend to produce more leaves than they need, and benefit from drastic pruning to thin the foliage and encourage good ventilation.

Once tomatoes start to flower, use only fertilisers that are high in potassium, such as dedicated tomato feeds. A high-nitrogen fertiliser will boost leaf production, making blight more likely.

Keep greenhouses or polytunnels well ventilated to stop them becoming too humid and to increase air flow.

Avoid planting tomatoes near potatoes

Don't plant tomatoes near potato plants, 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 larger ones such as beefsteaks, because they ripen more quickly and are often harvested before blight hits.

Practice good hygiene

Once blight hits, remove and destroy the affected foliage to reduce chances of further infection.

New blight strains in the UK may be able to produce resting spores that remain on plant material through winter and could reinfect next year's crop. Burn the waste, bury it underground in a deep hole or add the material to your green bin.

Don’t leave potato tubers in the ground at harvest as they, too, could harbour blight.

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.

Tomato 'Berry'

This unusual cordon tomato produces strawberry-shaped cherry tomatoes which have a super-sweet flavour. It’s early to ripen, and shows some resistance to blight.

• Buy tomato 'Berry' from Marshalls

Tomato 'Fandango'

A blight-tolerant cordon tomato that's reliable and resistant to verticillium wilt. It produces a heavy crop of medium-sized, deep-red fruit with an excellent flavour.

• Buy tomato 'Fandango' from Crocus

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.

• Buy tomato 'Fantasio' from Suttons

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.

• Buy tomato 'Ferline' from Thompson & Morgan

Tomato 'Latah'

Bush variety 'Latah' produces large cherry tomatoes with excellent flavour, and usually crops early enough to avoid blight.

Tomato 'Legend'

Bush variety 'Legend' holds the RHS Award of Garden Merit and has some blight tolerance. A salad type with heavy crops, it produces almost seedless fruit with superb flavour.

• Buy tomato 'Legend' from Thompson & Morgan

Tomato 'Lizzano' F1

A vigorous semi-determinate 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.

• Buy tomato 'Lizzano' from Suttons

Tomato 'Losetto' F1

This very blight-tolerant semi-determinate variety produces masses of bright red, sweet cherry tomatoes. 'Losetto' is ideal for growing in pots.

• Buy tomato 'Losetto' from Thompson & Morgan

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.

• Buy tomato 'Red Alert' from Suttons