Why do tomatoes get black on the bottom (blossom end rot)?

black on the bottom of tomatoes - blossom end rot

It feels like I invest a lot of time, energy and emotion into growing and ripening tomatoes. So, it is always a worry to see a tomato plant looking diseased or distressed in some way. 

It is especially concerning when the developing fruit are affected, because the fruit are the reward for all your hard work.

Therefore, the appearance of black or brown spots on the bottom of the tomato fruit can be a bit alarming.

These black spots on the bottom of the fruit are called blossom end rot and they appear at the point where the flower petals attached to the plant before pollination – hence the name.  The good news is that blossom end rot is not fatal to the plant or the other fruit. So, why do tomatoes get black on the bottom – or blossom end – of the fruit? In short, it is because of a problem with the distribution of calcium in the fruit tissue, mainly associated with how the plants are watered. 

Read on for full details of the causes of blossom end rot, how you can prevent it and what to do with affected plants and fruit.

Appearance of blossom end rot

blossom end rot on tomato truss

The early signs of blossom end rot are watery spots on the blossom end. These rapidly turn brown or black, and the tissue can break down and become sunken. At this point the area of brown or black on the bottom of the tomato fruit begins to dry out and can look leathery or cracked.

Blossom end rot can appear on green, immature tomatoes, as well as on fruit that is ripening or has fully turned red. Blossom end rot appears most frequently on the earliest tomato fruit of the season. However, it can appear at any time because, as outlined below, the calcium deficiency is associated with dry periods, root damage or uneven watering.

Apart from blossom end rot in tomatoes, calcium deficiency can result in ‘tipburn’ in leafy vegetables, ‘brown heart’ in lettuces or cabbage,  and ‘bitter pit’ in apples. Blossom end rot can also affect watermelon and peppers.

Causes of blossom end rot

Calcium is an essential plant nutrient and is required by  plants to create the structures of cell walls and membranes. Although calcium deficiency is rare in plants in nature, it does occur more frequently in plants in cultivation, usually when calcium uptake is temporarily interrupted.

Calcium is taken up from the soil through the plant’s roots. But it can only be transported successfully around the plant when there is sufficient moisture in the soil. When there is,  water can be drawn up into the plant to act as the medium for moving the calcium around the stems and leaves. 

When the plant is actively growing, a continuous supply of calcium is needed. When insufficient calcium is available for a continuous supply, then the calcium settles in one place  – on tomatoes, this as at the blossom end of the fruit – and causes the discolouration and rot.

In practice there are 5 main reasons why tomatoes get blossom end rot:

  1. Dry conditions or insufficient or inconsistent watering  – this can mean that there is not enough moisture to allow the calcium to be taken up through the roots of the tomato plant.
  2. Very wet conditions leading to water-logged soil – this can also be a problem, because water-logging affects the roots’ ability to absorb nutrients.
  3. Over-fertilisation leading to rapid growth of the plant – when rapid growth occurs, the calcium available cannot be taken up quickly enough to meet the demands of the plant’s growth.
  4. Damage to the tomato plant’s roots caused by cultivation (e.g. with a hoe) – obviously, if there are fewer or damaged roots, uptake of nutrients will be reduced.
  5. Naturally low levels of calcium in the soil.

How to prevent blossom end rot

As you might imagine, preventing blossom end rot involves taking steps to stop the typical causes of blossom end rot occurring. 

So here is how you stop your tomatoes going black on the bottom:

  1. Grow you plants in well-drained fertile soil with plenty of organic matter dug in. The drainage will prevent water-logging and the organic matter will help keep the soil moist;
  2. Ensure that you water your tomato plants  regularly and consistently so that you keep the moisture in the soil around your tomato plants;
  3. Give plants that are grown in garden beds about an inch (2.5cm) of water per week;
  4. Water your plants at the roots, rather than from above so that the water reaches the roots where it is needed, and also to prevent the spread of blight or other fungal diseases;
  5. Regularly check the moisture content of the potting soil of your container grown tomatoes, because it will dry out quickly in warm conditions;
  6. Mulch around your tomato plants with organic mulches like straw, wood chips or grass clippings in order to conserve moisture;
  7. Take care not to damage the tomato plants’ roots when digging, hoeing or tilling nearby;
  8. Grow your tomatoes in fertile soil with a a PH of 6.5 to 7.5;
  9. Use low nitrogen fertilisers that are higher in phosphorous and potassium.
drip irrigation of tomato plants
A drip irrigation system is the perfect way to ensure consistent watering of your tomato plants

What to do if your tomatoes have blossom end rot

When  you see the tell-tale signs of black on the bottom of the tomato fruit, there is no need to panic. 

Unlike fugal diseases like blight or septoria leaf spot (which starts affecting the lower leaves of the plant), blossom end rot does not spread from fruit to fruit. As we have seen, it is caused by the settling of calcium in one place on the fruit, so is not strictly a systemic disease of the plant.  

Therefore, affected fruit simply need to be removed and disposed of. Fungicides or other sprays are not needed.

If the conditions that caused the disruption of calcium intake are dealt with, then the continued development of the plant and your tomatoes should be unaffected.

I have seen blossom end rot affect just one or two tomatoes on a truss, with no obvious harm to growth or taste of the remaining healthy fruit in the tomato crop. 

unblemished tomatoes alongside one with blossom end rot
Unblemished tomatoes alongside one with blossom end rot

Blossom end rot FAQs

What is the fastest way to add calcium to soil?

You can use organic calcium, such as crushed  eggshells, ground oyster shells or dolomite lime. Alternatively, you can use calcium carbonate (but beware this will raise soil PH), wood ashes (which will also raise PH levels), gypsum (which is PH neutral), or bone meal.

Bone meal and eggshells are relatively slow in acting, so they may not be what you want if you need a quick solution.

dolomite lime for calcium for tomatoes
Add dolomite lime to the soil for calcium for tomatoes

Does Miracle Gro help blossom end rot?

Not really. The critical step if blossom end rot occurs is to ensure the plant is thoroughly and consistently watered. Unless there are other signs of nutrient deficiency, such as yellowing of the leaves between the leaf veins, then fertiliser is not usually the answer.

Most Miracle Gro products are high in nitrogen and this may induce blossom end rot in young plants if they grow too rapidly.

How do I add calcium to my tomato plants?

See the answer above in relation to the different calcium containing treatments. Mostly, you add these products to the soil and water in. If you are using a commercial product follow the instructions on the packet.

Can I pour milk on my tomato plants?

Milk contains calcium and it can be used on tomatoes. Use low fat milk and dilute with water (a 50/50 mix). However, too much milk can cause problems because of the bacteria in the milk. I think there are better options, so I don’t personally use milk in this way.

Does baking soda help blossom end rot?

I’ve never used baking soda to help with tomato blossom end rot and I haven’t found any reliable information to convince me that it does. 

There is some evidence that in a dilute solution it might help against fungal diseases, such as powdery mildew and blight.

What is the best product for blossom end rot?

The best product for blossom end rot falls out of the sky or pours out of your tap – water.  If your tomato plants get blossom end rot, remove the affected tomato fruit and maintain a consistent watering regime.

If, through a soil test, you determine that your soil has a particular nutrient deficiency or problem with soil ph, then add what is needed according to instructions.

If there is no obvious sign of nutrient deficiency then, when you plant tomatoes add well rotted compost to your soil. If you practice consistent watering and cover the soil with an organic mulch, you shouldn’t be much troubled by the black spots on the bottom of your tomato fruit again.

irrigation for tomato plants
Water is the best ‘product’ for preventing blossom end rot on tomato plants

References

White, P. J., & Broadley, M. R. (2003). Calcium in plants. Annals of botany, 92(4), 487–511. https://doi.org/10.1093/aob/mcg164 

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