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Tomato blight: what can you do about the disease?

Brown spots on tomato plants? The fruits rot on the stem? The leaves are turning brown? Perhaps it is tomato blight. Although plants affected by this disease die relatively quickly, you can still save some of your tomatoes thanks to these tips.

Recognize blight on tomatoes

when to spray tomatoes against brown rot

Late blight is a disease caused by the fungusPhytophthora infestansand can affect both tomatoes and potatoes. Late blight can occur at any time during the growing season, especially when the weather is cool and wet. Prolonged hot dry spells can slow, but not stop, the spread. Both green and ripe tomatoes can be infected.

why do tomatoes get brown rot

The pathogen not only destroys the fruit of plants it infects, but can also spread very quickly and widely, wreaking havoc in your garden if not properly controlled in a timely manner. Worst of all, blight can overwinter, meaning it can reappear the next year even after a cold frost if the plants and soil containing the pathogen are not removed.

It is difficult to detect blight in tomato plants in the early stages of the disease. Many growers do not immediately recognize that the crop is infected. If brown spots appear on the leaves, this can be confused with leaf spot disease(Alternaria tomatophila).


This is how you can recognize tomato blight:

  • Leaves have large, dark brown spots with a green-gray edge that are not bordered by the main veins of the leaf.
  • These spots increase in size until the entire leaf or stem turns brown and dies. Dead leaves usually remain attached to the stems.
  • Dark brown, irregular spots cover large portions of the fruit. The spots may become mushy as secondary bacteria invade.
  • A thin, white mold coating forms on infected leaves, fruit, and stems when humidity is high.

It is not uncommon for tomatoes to become damaged internally as well. In this case, the disease develops without visible signs. The buds ripen quickly and fall off. A lesion on the flesh of the fruit is discovered only when the vegetable is cut open.

Blight – is it possible to eat the tomatoes?

How to recognize brown rot in tomatoes

After the discovery of late blight in tomatoes, gardeners wonder if ripening tomatoes are still edible. Opinions are divided on this.

“The parts that are not infected are probably safe to eat. Tomato parts without disease symptoms probably do not pose a health risk to consumers,” says Dr. Margaret McGrath, a plant pathologist at Cornell University and a specialist in the disease.

However, Barbara Ingham, food safety specialist at the University of Wisconsin-Extension , advises consumers to resist the temptation to eat diseased tomatoes or potatoes.

“Since eating fruit affected by late blight has not been shown to be harmful, it may be tempting to simply cut off the affected part. But the fruit then tastes bitter and may harbor other organisms that can cause food infections.

which tomatoes are resistant to brown rot

Ingham also points out that diseased fruit, even if the infected part has been removed, should not be canned or frozen. “Don’t be tempted to pickle or preserve infected tomatoes,” Ingham says. “The virus can alter the acidity of tomato fruit, which is critical to safely preserving tomatoes. However, healthy tomatoes can be safely canned or even frozen,” she says.

What if you have undamaged tomatoes growing on plants whose leaves, stems or neighboring fruits show signs of infection? Those can be safely eaten and even preserved, Ingham says.

Why do tomatoes get blight?

prevent late blight in tomatoes

Tomato blight is most commonly caused by the following reasons:

  • The disease often manifests itself actively with an intense change in weather. If one month is cool and rainy and the next is hot and dry, the disease is bound to occur.
  • Poor soil. Sometimes gardeners don’t even know what the pH of the soil on their property is. If the pH is too high or too low, tomatoes will rot.
  • Calcium deficiency is a problem for most gardeners. The experienced gardener knows that tomatoes need to be fertilized with calcium nitrate during their growing season to prevent blight.
  • Nitrogen fertilizer in large amounts will cause late blight.

To protect your vegetable garden from disease, you need to know how to deal with these problems and how to prevent tomato blight.

What to do about tomato blight?


So what should you do if you discover late blight in your garden? Unfortunately, you can’t “cure” this disease, but you should uproot and remove plants with late blight symptoms immediately. By doing so, you will protect your neighbors’ gardens and local farmers.

What you should do:

1. remove the infected plants: remove infected plants, pack the foliage and immature tomatoes in black garbage bags and dispose of them with household waste. Do not attempt to compost infected potato or tomato plants. Most compost piles are not hot enough to kill the spores, and you risk re-infecting your plants the next season. You can read more about this here .

tomato diseases brown rot

2. Prevent spores from overwintering: Late blight can only survive on living tissue. Winters in the north kill any spores that may be above ground on trellises and tomato cages. However, late blight can overwinter in plant debris, seeds and potato tubers below the frost line. To be on the safe side, remove all plant debris at the end of the season, buy fresh seed for next year, and remove all potato and tomato plants that sprout from the ground.

3. next year, make a fresh start: At this point, all we can do is cut our losses and hope for a better growing season next year.

Fighting blight on tomatoes

what to do when tomatoes have brown rot

In the fight against this disease, you can use several proven methods to prevent its spread.

Soda against brown rot

At the first signs of damage to the plants, it is recommended to spray the leaves with a 1-2% solution of sodium bicarbonate. To do this, dissolve 10 to 20 grams of soda in 10 liters of warm water.

Use of wood ash

use of wood ash on tomatoes

Remove all infested fruits and leaves and treat the remaining plants with ar wood ash decoction. The ash contains phosphorus, calcium, magnesium and boron and also acts as a fertilizer . How to prepare wood ash broth:

  • Add 100 grams of ash to 10 liters of boiling water.
  • Then let the mixture cool and then strain it.
  • Pour the solution into a plant sprayer and treat the fruits, leaves and roots.

what to do with tomatoes brown rot

Calcium chloride and calcium nitrate

External application of calcium chloride and calcium nitrate can correct calcium deficiency and stop the development of rot lesions. To prepare the solution, you will need:

Variant 1.
Calcium nitrate – 5 to 10 grams
1 liter of water

Variant 2.
Calcium chloride – 30 to 40 grams
10 liters of water

First, you should moisten the soil, and then spray the tomato plants with the calcium solution.

Superphosphate fertilizer strengthens plants.

Superphosphate fertilizer strengthens plants

Additives containing phosphate help to stop the development of blight and prevent it from spreading to the healthy bushes. Superphosphate has proven to be effective. To prepare the base you will need:
3 liters of water
15 to 20 tablespoons of superphosphate.

Since the fertilizer dissolves more easily at high temperatures, bring the water to a boil and pour in the granules. Stir the solution occasionally during the day until the granules are completely dissolved. In this way, you will get a mixture with a light texture.

To obtain the remedy for the treatment of tomatoes against blight, add 150 ml of the basic solution in 10 liters of water and add 20 g of nitrogen fertilizer as a necessary additive.

Prevent tomatoes blight

Tomato varieties resistant to blight

The best control of late blight on tomatoes consists mainly in the preventive measures. This is essentially a matter of:

  • selecting resistant tomato varieties
  • disinfect the seeds before sowing
  • prepare the soil before planting the plants
  • to pay attention to regular watering

If the tomato varieties are resistant to blight, this is usually indicated by the manufacturer on the packaging. The most resistant tomato varieties are:

  • Philovita
  • De Berao
  • Resin fire
  • Phantasia
  • Mountain Magic (F1)
  • Plum Regal (F1)
  • Cocktail tomato Philovita (F1)
  • Mountain Merit (F1)
  • Tomato Fantasio (F1)
  • Heaven Striker
  • Legend
  • Wapsipinicon Peach
  • Lemon Drop
  • Pruden’s Purple

seeds in potassium permanganate

The fight against pot rot of tomatoes begins with disinfection of seeds before sowing. If the seeds are store-bought, this treatment is usually not necessary. If you use your own harvested seeds, you can disinfect them with potassium permanganate. Soak the seedlings in a 3% solution of potassium permanganate for about half an hour, then wash them under warm running water and dry them.

Proper preparation of the soil before planting tomato plants also helps to protect the fruit from blight. To do this, experts recommend adding 2 to 3 tablespoons of fine lime or ordinary chalk to the soil.

Wood ash as fertilizer when planting tomato plants in the ground

Using wood ash as fertilizer when planting tomato plants in the soil also helps to prevent blight from affecting the fruit. To prevent the disease, add a coffee cup of ash to each planting hole.

When growing tomatoes, it is important to provide timely irrigation. Drought reduces the calcium supply to the plant, which can lead to blight. Since calcium is better absorbed at night, it is advisable to water in the evening. A layer of mulch will help retain moisture in the soil longer.

Tomato blight fungicides

Tomatoes blight fungicides

Fungicides can help protect healthy, uninfected plants if they are sprayed regularly on all parts of the plant. A copper fungicide is less effective, but probably the best choice for organic gardeners. In all cases, you must apply the fungicide prior to infection for it to be effective. Always follow label directions when applying fungicides.

  • Organic fungicides. Treat plants with a copper fungicide that you can buy online, at a hardware or home improvement store. Follow the instructions on the label. You can apply it once a week and after each rain until the leaves drip off. The biofungicide group also includes Serenade® ASO.
  • Chemical fungicides. Some gardeners prefer chemical fungicides, such as Ranman and Signum, which effectively inhibit fungal development. You can also opt for pesticides containing the active ingredient mancozeb, although these have a longer waiting period before you can safely harvest tomatoes.

Once late blight arrives and progresses, it becomes increasingly resistant to biofungicides and fungicides. Treat the plants as soon as possible and according to a specific schedule.