Tomatoes Turning Black: 7 Common Causes and Fixes


You’ve planted your tomatoes, and it feels like you’ve done everything right. But then, you start to notice that some of them are turning black. Although it’s disheartening to see your tomatoes lose their color, this is a pretty common problem that you can fix.

Tomatoes can turn black because of calcium deficiencies, fungal infections, too much salt in the soil, or sun scald. Some common fixes include using fertilizers, reducing contact between tomatoes and the ground, and protecting the tomatoes from the sun.

This article goes through the common causes behind your tomatoes turning black. I also offer some tried-and-tested fixes to these problems. Let’s get into it.

1. Blossom End Rot

Blossom end rot is a common disorder that causes the end of the tomato to break down and rot. It is not caused by any organisms. The main culprit behind blossom end rot is the lack of calcium in the plant.

Calcium is one of the most important micronutrients for tomato growth. It is used to form new cells for roots, stems, and fruit. It holds the tissues together to maintain healthy growth.

Two major calcium problems create blossom end rot in tomatoes:

  • The plant is growing too fast. When this happens, the plant can’t take up enough calcium to support the rate of tomato growth. This is usually due to too much fertilizer.
  • Certain conditions are stopping the plant from processing calcium. These include calcium shortages, pH imbalances, inconsistent watering, or lack of calcium in the soil. 

Lack of Calcium in the Soil

The amount of calcium in the soil is sometimes determined by the soil type. Light, coarse, acidic soil tends to have less calcium than heavy, loamy soil. If the earth has relatively little water content, it most likely has little calcium.

The soil might also lack calcium when the temperatures are low, such as in the early months of springs. If the tomatoes grow in soil with high clay content, the calcium deficiency may be more evident. Clay can stop the roots from spreading out, reducing access to more nutrients, including calcium.

How To Fix Low Soil Calcium

Agricultural lime is the front-line solution to improving calcium levels in acidic soils. As you add the lime, take note of the increase in pH, with a target of 6.5 in mind.

Adding agricultural lime to acidic soil is a good way to solve calcium deficiency. Unless the plants prefer acidic soil, you should aim at a pH of 6.5.

Lack of Calcium in the Plant

In other cases, the soil might have healthy calcium levels, but the plants fail to absorb it. 

How To Fix Low Plant Calcium

The first action required to give your plants enough calcium is getting the pH of the soil right. The tomatoes will quickly absorb most calcium if the soil remains within the 7.5 to 9.5 range. 

Add elemental sulfur or dilute sulphuric acid to lower the pH of the soil if it is above this pH bracket. If you have slightly acidic soil, use lime to increase the pH.

If you notice that calcium levels are low in spring, try warming the soil. Use mulch if the plants are already in the ground. If it’s not yet planting season, apply a black plastic sheet over the earth. You can also warm the soil by putting it in greenhouses.

Inconsistent Watering

If your watering patterns are erratic, you may stress the tomatoes, which causes poor movement of calcium to the plant and can cause blossom end rot. 

Calcium requires water to move from the soil to the roots and the rest of the plant. It won’t directly transfer from the leaves to the tomato, making the absorption from the ground a critical step.

When you don’t water the plant regularly, it prioritizes the movement of the calcium to the leaves and not the fruit. When the fruit is not supplied with enough calcium, the cells won’t bind and grow properly. This causes the tomato tissue farthest from the stem to rot.

You may also notice a calcium deficiency when there’s a drought that causes unpredictable moisture levels in the soil. 

How To Fix Inconsistent Watering Problems

It’s essential to draw up a consistent watering schedule. Ideally, water the plant twice a day, early morning and evening. 

If you miss a day or session, do not compensate by overwatering the tomatoes the next time. In addition, make sure your drainage is good enough not to hold water for long. The rule of thumb is to water less frequently but allow the water to go deep.

Using Excessive Fertilizer

Excessive fertilizer can lead to blossom end rot, especially when using a nitrogen fertilizer. Nitrogen fertilizer encourages rapid growth of the shoots, which prematurely depletes the calcium in the soil.

Ammonia-based nitrogen fertilizers interfere with the way the tomatoes absorb and transport calcium.

How To Fix Excessive Fertilization

Use fertilizers that are low in nitrogen and high in phosphorus and calcium to avoid problems with calcium uptake. Secondly, moderate the amounts of fertilizer that you use to prevent abnormal growth spikes in your tomatoes. You should aim for steady growth over a long period.

pH Imbalance

Tomatoes absorb nutrients depending on availability in the soil. The type of nutrient that your plant takes up depends on whether or not the soil pH favors its availability.

For example, acidic soils favor the availability of iron and aluminum. As the roots of the tomato plant absorb the nutrients, less calcium will be taken into the stem, leading to a calcium deficiency in the long haul.

How To Fix a pH Imbalance

Create a balanced pH for your soil by using agricultural lime for acidic soil. The best time to apply the lime is a few months before the planting begins. 

It can be finely ground, granular, pelletized, or hydrated limestone. The rule of thumb is that the finer the limestone, the more quickly it changes the soil’s pH. 

2. Buckeye Rot

Tomatoes may blacken due to a disease called buckeye rot. It is caused by different species of the microorganism Phytophthora. The condition can lead to massive losses in the harvest if its spread is not controlled early in the season.

Buckeye rot is usually seen when temperatures are warm, and moisture is high. It is also prevalent in high humidity areas that receive plenty of rain. Between 75 and 86 °F (24 and 30 °C), infected tomatoes will start rotting within 24 hours. The pathogens reproduce fast in wet soils above 65 °F (18 °C).

It is common to see a breakout of buckeye rot after a heavy period of rainfall or intensive irrigation. The sudden water saturation stimulates the release of disease-causing spores from the ground onto the surface. Buckeye rot spreads when tomatoes near the ground get into contact with infected soil. 

The fungal disease usually comes from spores of infected seeds or plants from the previous season. It spreads to the tomato when the soil makes contact with the tomato. This can be when the tomato touches the ground or when dirt splashes on the tomato’s skin. 

It starts as a small, brown water-filled spot on the green or ripe fruit—the lesion forms at the point of contact between the infected soil and the tomato. Over time, the spots dig into the tomato with alternating light and dark rings. 

The rings are smooth and do not form any raised margins. This is one of the features that distinguishes buckeye rot from late blight caused by Phytophthora infestans. The infected tomato stays firm, although it may become flabby due to later infection from soft rot bacteria.

When the disease is left untreated, the tomato becomes rough, and it darkens over time, giving it a black patch.

How To Fix Buckeye Rot

Currently, there are no disease-resistant varieties. You can prevent and control the symptoms of tomato buckeye rot using these techniques:

  • Discard infected fruit from the plant and the garden during and after the growing season. Any infected tomatoes left on the plant will allow the pathogens to multiply. Infected tomatoes will probably fall off, leaving the disease pathogens to stay in the ground. When the harvest is done, get rid of all tomato vines by burning them.
  • Use fungicides regularly on the infected tomato. The most effective fungicides for tomato buckeye contain metalaxyl or chlorothalonil. Other active ingredients that work well include maneb and mancozeb.
  • Create good soil drainage. If your soil holds water for abnormally long periods, it becomes a breeding ground for fungal diseases. 
  • Use raised beds to prevent spread from one area to another. You may also fumigate the soil if you believe the garden is infested with tomato buckeye rot.
  • Shift your garden to another spot if the infection has spread widely. You might want to leave the infected land uncultivated for a couple of years. 
  • Carry out crop rotation. Skip a few seasons before you plant tomatoes again. During this time, grow plants outside the tomato family.
  • Reduce contact between the soil and the tomatoes. The best way to do this is by mulching. This forms a barrier that prevents dirt from splashing onto the healthy fruit.
  • Avoid doing any hands-on work on the plants when the foliage is wet. This could involve not performing pruning or harvesting after a downpour or dew. The spores tend to spread faster under wet conditions.

3. Anthracnose

This is a fungal disease that can cause tomatoes to darken. It is caused by a species of fungus called Colletotrichum. The disease primarily affects ripe and overripe tomatoes. It can affect unripe fruit too, but it does not show any signs until the tomato ripens. 

Anthracnose typically develops in temperatures between 12–27℃. It spreads faster in wet conditions where dew or rain lasts for more than 12 hours a day. It’s also commonly found where overhead irrigation is practiced.

The fungal disease causes small, deep spots between a quarter to half an inch in diameter. The spots become darker and more prominent over time and may form rings as the disease progresses. 

You may notice a dark color at the center of the spots as structures that hold spores start to form. Sclerotia can stay in the soil and lead to infections even after three years.

During periods of high humidity, pinkish spores are released from the sclerotia. 

In the long term, anthracnose may attract more organisms that cause the entire fruit to rot. 

Anthracnose spreads from one fruit to another when rain splashes the spores between tomatoes. The tomatoes on the ground are usually at most risk of infection. 

How To Fix Anthracnose

You can control the spread of anthracnose by:

  • Discard infected fruit from the plant and the garden during and after the growing season. When the harvest is done, get rid of all tomato vines by burning them. It is not safe to add them to compost because they may infect the soil again.
  • Use fungicides regularly on the infected tomato. The most effective fungicides for tomato buckeye contain metalaxyl or chlorothalonil. Other active ingredients that work well include maneb and mancozeb.
  • Create good soil drainage. If your soil holds water for abnormally long periods, it becomes a breeding ground for fungal diseases. 
  • Use raised beds to prevent spread from one area to another. You may also fumigate the soil if you believe the garden is infested with tomato buckeye rot.
  • Shift your garden to another spot if the infection has spread widely. You might want to leave the infected land uncultivated for a couple of years. 
  • Carry out crop rotation. Skip a few seasons before you plant tomatoes again. During this time, grow plants outside the tomato family.
  • Reduce contact between the soil and the tomatoes. The best way to do this is by mulching. This forms a barrier that prevents dirt from splashing onto the healthy fruit.
  • Avoid doing any hands-on work on the plants when the foliage is wet. This could involve not performing pruning or harvesting after a downpour or dew. The spores tend to spread faster under wet conditions.

4. Bacterial Spot

Bacterial spot is a disease caused by Xanthomonas vesicatoria, Xanthomonas euvesicatoria, Xanthomonas gardneri, or Xanthomonas perforans. The disease spreads through infected seeds or transplanted plants. The infected plants may not show any symptoms at times.

The tomato gets infected through tiny openings in the plant. In warm, wet weather, bacterial spots can spread if pathogens are blown by windy rain from one plant to another.

It can affect both green and ripe tomatoes. On green tomatoes, it creates small spots that look like blisters with a yellow ring around them. The spots increase in size and become rough and raised as the disease matures.

The symptoms of bacterial spot may sometimes be confused with those of another disease, bacterial speck.

How To Fix Bacterial Spot

Bacterial spots can’t be cured in infected plants. However, there are a few precautions you can take to stop the disease from breaking out:

  • Do not plant diseased seeds or transplants. This is the best way to avoid bacterial spots from attacking your crop.
  • If you are working with seed that you think is infected, soak it for half an hour in water at 50°C. This can prevent outbreaks.
  • Use a soaker hose or drip tape when watering your tomatoes. Overhead sprinklers will get the fruit wet and, therefore, more prone to infection.
  • Sterilize your garden tools before you use them. This can keep bacterial spot from spreading between plants.
  • After the season, burn any tomato debris or volunteer crop that comes up. That way, disease won’t spread from one plant to the next between seasons.

5. Sun Scald

Sun scald is a condition that causes yellow patches on ripe tomatoes and white patches on green tomatoes. With time, the patches turn into blisters that flatten the shape of the tomato at the infected spot. 

These patches may eventually turn to a grayish color, making the skin of the tomato rupture. When the rupture happens, black mold grows over the tomato. The infected area starts to rot and then spreads to other areas of the fruit.

As the name suggests, sun scald is caused by overexposure to the sun. It’s common in tomato plants with sparse foliage, leaving many of the tomatoes exposed to a lot of sunlight. Scalded tomatoes are prone to attack by other parasites and disease-causing organisms.

Here are some of the practices that cause this exposure to happen:

  • Not providing shade to tomatoes that are constantly exposed to sunlight.
  • Poor pruning that leaves the tomatoes exposed to direct sunlight.

How To Fix Sun Scald

Tomato sunscald is one of the most manageable disorders to control. Do this to protect your tomatoes from being burned:

  • Plant tomatoes that are heat resistant. If you grow tomatoes in hot temperatures, steer clear of varieties that mature with little foliage.
  • Water regularly and sufficiently. Tomatoes are less susceptible to sun scald when they’re hydrated.
  • Cover your soil to maintain moisture in the ground and keep your tomatoes from drying up. Mulch also prevents water from splashing disease-causing dirt onto your tomatoes.
  • Only prune when it is necessary. Too much pruning can stress the tomatoes and increase the risk of sun scald.
  • Harvest tomatoes that are starting to ripen immediately. Let them fully ripen indoors.
  • Shade exposed tomatoes if they cannot be protected from the sun. This is especially important in hot, dry climates.

6. Too Much Salt in the Soil

If your tomatoes are turning black, one of the causes could be the excessive salinity of the soil. Soil salinity refers to the amount of water-soluble salts in the soil. 

When soil is too salty, it reduces the amount of water available for the plants to absorb. As a result, the tomato dies due to dehydration. 

It’s not that the tomato plant doesn’t take up the water. It’s just that when the water is absorbed, it has so many sodium ions that are forced out of the tomato cells through osmosis. This process can contribute to the darkening of the tomato.

There are numerous reasons why salt levels can be high in the soil:

  • Soil irrigated with saltwater.
  • Dry climates that do not allow much leaching of the salt.
  • Excessive use of fertilizers.
  • Poor draining of the soil.
  • High evaporation brings salt closer to the ground.
  • Geological leakages depositing salt into the ground.

How To Fix Too Much Salt in the Soil

If you suspect that your tomatoes are turning black because of the high salinity of the soil, try the following:

  • Reduce the use of salty water for irrigation; harvest rainwater or use desalinated water.
  • Use organic matter to contain moisture in the soil.
  • Improve drainage to allow movement of salts from the soil.
  • Cut down evaporation using mulch.
  • Apply fertilizers sparingly, especially nitrogen fertilizers.

7. Cold Air and Soil

The temperature of the soil is an essential factor in plant growth. Besides playing a significant role in how plants receive nutrients from the ground, it guides farmers on when to plant seedlings. 

The temperature of the soil varies each day, month, and season. As it changes, so does the quality of plant growth. Cold soils usually slow down the uptake of nutrients by the plant. In addition, when the temperatures are low, there is a decrease in the microbe activity of decomposition of organic matter.

These dynamics combined can significantly affect the amount of nutrients that the tomato gets. If the tomato tissue lacks certain minerals, especially calcium, they may stop growing. Dead cells are alienated from the tomato and tend to darken with time.

How To Fix Cold Air and Soil

Keep the soil warm by applying a black plastic sheet over. It will lock in moisture while preventing weeds from sprouting. Plastic is okay to use, provided you don’t leave it in the soil at the end of the planting season. 

If you are skeptical about using plastic, you may use black biodegradable plastic mulch (BDM). BDMs can safely remain in the soil when it’s time to till the bed. Over a period of about two years, they are broken down by microorganisms in the soil. 

It’s important that the film of mulch be black because the color easily absorbs and retains heat from the sun. Apply the sheet onto the soil and leave it for about three weeks. You can either take it off before planting the tomatoes or cut holes in it where the tomatoes will grow.

Alexander Picot

Alexander Picot is the principal creator of TheGrowingLeaf.com, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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