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Blight in tomatoes: What are the causes and means of prevention and control of the fungal disease?

Blight in tomatoes can affect almost all parts of the plants, including the leaves, stems and fruits. The plants do not die, but they are weakened and set fewer tomatoes than normal. Stressed plants or plants in poor health are especially susceptible. The fungal infection can cause a plant to die within a week. Therefore, it is important to watch for telltale signs and act quickly as soon as you spot them.

What causes the disease

What causes blight in tomatoes, learn from us

Blight is a common tomato disease caused by the fungus Alternaria solani. The fungus can come from many sources. It can be in the soil or already on purchased seeds or seedlings. It can even overwinter in the diseased remains of your plants and persist in the soil or remains for at least a year. Blight can occur in any weather, but it prefers moist conditions, such as frequent rain or even heavy dew.

Symptoms of the fungal disease

Check tomato plants regularly during the growing season

In tomato blight in the early stages, black rings first appear on the lower leaves. This causes the leaves to turn pale green to a musty yellow. Leaves eventually become so severely infected that they fall off.

The fungal infection can affect almost all parts of the tomato plants

On older plants: Dark spots with concentric rings form first on older leaves. The surrounding leaf area may turn yellow. Infected leaves may die prematurely, exposing fruit to sunburn. The dark lesions on the stems are initially small and slightly sunken. As they grow larger, they expand and concentric markings like the spots on the leaves are seen.

In tomato blight in the early stages, black rings first appear on the lower leaves


On tomato fruit: When blight spreads to fruit, the spots start at the stem end and form a dark, leathery, sunken area with concentric rings. Both green and ripe tomatoes may be affected.

On seedlings: Affected seedlings have dark spots on leaves and stems. They may even become diseased on the cotyledons. The stems often become garland-shaped.

Measures for prevention

There are a number of varieties that have been bred with resistance to the infection

Blight in tomatoes cannot be prevented entirely – but you can take steps to reduce the likelihood of infection. You can control the problem with the following measures:

  • Certified seed: buy seeds and seedlings from reputable sources and test all plants before placing them in your garden.
  • Air circulation: make sure you have adequate space for plants. Good air circulation will help keep the plants dry. If you are growing tomatoes in a greenhouse or polytunnel, make sure they don’t get too moist by keeping them well ventilated. Also, choose a well-ventilated place to grow tomatoes outdoors. To keep plants dry, it is important to prune leaves and side shoots to maximize air movement and ensure evaporation of surface moisture. Remove the lower leaves as the fruit develops. This will prevent the leaves from resting on the moist soil. Air circulation is encouraged and fruit ripening is accelerated as they receive as much sun as possible.

If the infection breaks out in your area, you should plant your tomatoes elsewhere the next year


  • Garden hygiene: since blight can overwinter on plant debris and in the soil, thorough cleaning of the garden is essential.
  • Fruit rotation: If you have an outbreak of the fungal infection, you should plant your plants elsewhere next year, even if they are in containers.
  • Tie or stake the plants: Support bush – plants with a stake to keep their leaves off the ground, or tie cordon tomatoes regularly with soft twine to improve air circulation. Shrub tomatoes grown in pots can be partially supported with a bamboo cane.

The fungus can overwinter in the diseased remains of your tomato plants

  • Water the plants in the morning. This will prevent them from staying damp all night. Try to water only the soil and keep the leaves dry.
  • Never plant tomatoes in soil or compost that previously contained diseased plants. Tomato disease spores can remain in the soil for 3 to 4 years.
  • Proper composting of plant debris is of utmost importance, and gardens and allotments should be free of “plant debris” to reduce infection.

Grow varieties that are resistant

You can grow tomatoes that are resistant to the fungal disease

Many modern varieties have made tomatoes easier to grow through earlier maturity and improved disease resistance. There are a number of varieties that have been bred with blight resistance.

  • ‘Merrygold’. This is the world’s first blight-resistant variety with orange fruit. This plant is very productive and produces a lot of bright orange tomatoes.
  • ‘Cocktail Crush’. This variety is very resistant to blight and is ideal for outdoor growing.
  • ‘Crimson Blush’. This mold-resistant beefsteak variety is sweeter than other beefsteak varieties.
  • ‘Tommy Toes’ are red, tiny heirloom tomatoes that have a strong flavor and are very juicy.
  • ‘Legend’ is a red beefsteak variety and a favorite of most commercial growers.
  • ‘Juliet’ is a small red hybrid variety.
  • ‘Manyel’ is a yellow tomato that is also low in acid.

Fighting blight in tomatoes

What to do about blight in tomatoes? If the worst happens and you notice signs of the infection on some of your plants, the best treatment is to remove those infected plants immediately to prevent spread to other plants. Check plants regularly during the growing season, especially if conditions are warm and humid. If possible, burn infected plants or compost the foliage and fruit if you have a good composting system, as composting kills the spores.

Both green and ripe tomatoes can be affected by the fungal infection

It is important not to plant your tomatoes next to potatoes, which can also be affected by blight, but to choose their companion plants carefully .

Blight tomato spray: baking soda.

Baking soda can be used as a treatment for the infection. You can make the following solution:

Add to 4 liters of water:

  • one heaping tablespoon of baking soda;
  • a teaspoon of vegetable oil;
  • a small amount of a mild soap.

Spray the plants with this solution and apply it regularly to maintain its effectiveness.

Are blight tomatoes edible

If you have an infected plant, you should not eat fruit that is obviously affected by the fungal infection, but ripe fruit that shows no signs of blight is still perfectly edible. However, if you leave fruit on an infected plant, it will not ripen. You can harvest green tomatoes that show no signs of blight and make them into green tomato chutney.

Can blight tomatoes be edible if they show no sign of fungal infection