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How to Prevent Blossom End Rot

Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.

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You’ve been carefully nurturing your tomato plants all summer and finally they are setting fruit. But, wait. What’s that soft black spot on the bottoms of your tomatoes? You’ve got blossom end rot.

What is Blossom End Rot?

Blossom end rot is not a disease. It is a symptom of a lack of calcium most often seen in the first fruits produced by your plants. It is characterized by a softening of the bottom, or “blossom end” of the fruit. The brown or black that you see is actually a fungus that is taking advantage of the fruit’s weakened condition. The fungus does not cause the rot. It is a naturally occurring organism that doesn’t infest strong, healthy fruit. It only occurs in fruits that are weakened by damage, disease or lack of nutrients.

Blossom end rot is not confined to tomatoes. It also appears in peppers, eggplants and squash.

What Causes Blossom End Rot?

Blossom end rot is caused by a lack of calcium. There are three causes for the lack of calcium in your plants.

  1. Your soil can be lacking in calcium.
  2. Calcium is present in your soil but not in a form that your plants can use
  3. Inconsistent watering.

A soil test will tell you if your soil is low in calcium. Or you may have calcium in your soil, but because your soil is acidic, plants can’t access the calcium so you will need to adjust the pH of your soil to make it more alkaline.

Too much or too little water will prevent your plants from being able to extract calcium from your soil. Uneven watering or periods of rain followed by periods of dryness make it difficult for plants to get vital nutrients from the soil.

Why is Calcium Important to Prevent Blossom End Rot?

Calcium is important for normal cell growth in plants. It helps the cells regulate their internal processes. Calcium also helps plants resist insects. Insects are more likely to infest unhealthy plants rather than strong, healthy plants with plenty of calcium.

An important thing to remember about calcium in plants is that it doesn’t move through the plants. Once a plant takes up calcium from the soil to grow a leaf, it cannot move that calcium to another branch to grow another leaf. The plant needs to continually absorb calcium from the soil as it grows and sets fruit. If it is unable to consistently absorb calcium, it will not be able to produce more foliage and fruit.

Often the first fruit (left) is affected by blossom end rot because the plant is growing so quickly.  Subsequent fruits (right) have no rot.

Often the first fruit (left) is affected by blossom end rot because the plant is growing so quickly. Subsequent fruits (right) have no rot.

Why do Only the First Tomatoes Have Blossom End Rot?

Even if you soil is fine and you are consistently watering your tomatoes, blossom end rot can appear in the first tomatoes of the season. That is because at the stage, the plants are still growing rapidly and are unable to absorb enough calcium to fuel that growth. Once the plants have set their first fruit, their growth slows and their need for calcium drops. They are able to absorb enough calcium from the soil to both grow and set fruit so the subsequent tomatoes have no blossom end rot.

If you see blossom end rot on your first tomatoes, remove them from the vines and dispose of them. If the next crop of tomatoes also has rot, then you will need to take steps to prevent it.

How to Prevent Blossom End Rot

Get A Soil Test Done

The most important step you can take to prevent blossom end rot is to have your soil tested by a professional soil lab. The reason why you want your soil tested by a soil lab rather than using a kit that you purchase at the store is that the lab will test your soil for all nutrients and minerals. The report they send you will tell you exactly how much of each mineral is in your soil and how to amend your soil for the optimal mix of nutrients. A soil test kit that you purchase from the store will only test for a few minerals and may not tell you how to amend your soil.

The best time to have a soil test done is in the fall. That way you will be able to add your amendments and have the entire winter for them to be incorporated into your soil. Your soil will be ready for planting in the spring.

Amend Your Soil

Tomatoes grow best in a soil with a pH of 6.5 to 7. If your soil is too acidic (below 7), you will need to add lime to bring the pH level up. The soil test report will tell how much lime you will need to add to your soil to bring it to the proper level. If your soil is too alkaline, you will need to add gypsum to bring the pH level down. Use the soil test report as a guide to how much gypsum you need to add to your soil

I always like to add some crushed eggshells in the bottoms of the planting holes when I am planting my tomatoes, peppers, eggplants and squash to give my plants an extra boost of calcium. Wash your eggshells and then let them dry completely. You can either crush them manually using your fingers or a hammer. A rolling pin also works nicely. Or you can do it mechanically using an herb grinder or coffee grinder. Make sure to wash them thoroughly after use to avoid getting eggshell “grit” in your herbs or coffee!

Drip irrigation tubes used on a farm.  Home gardeners can install smaller systems in their gardens.

Drip irrigation tubes used on a farm. Home gardeners can install smaller systems in their gardens.

Use Drip Irrigation

Too much or too little water can prevent your plants from absorbing the calcium they need from the soil. Consistent amounts of moisture will make consistent amounts of calcium available to your plants. Drip irrigation, especially when used on a timer, is the perfect way to ensure that your garden has dependable amounts of moisture.

Sprinklers are an inefficient way to water. A significant amount of water evaporates before it ever reaches the soil. The water that does reach the soil hits it so hard that it often splashes up onto the foliage of you plants and encourages the formation of downy mildew. Drip irrigation releases water slowly into the soil without splashing it all over. Also, all of the water makes it into the soil. There is no chance for evaporation.

Putting your drip irrigation system on a timer means that it will go on every day for a set period of time. The same amount of water will be released into your soil every day. You don’t need to remember to water. If you have rain, you can always stop the timer so as not to over-water your garden.


A thick layer of mulch will keep your soil evenly moist and prevent weed seeds from sprouting

A thick layer of mulch will keep your soil evenly moist and prevent weed seeds from sprouting

Use Mulch

Mulch serves many purposes in the garden. The two most important are to help the soil retain moisture and suppress weeds. Mulching your garden is another way to ensure consistent moisture in your soil. A thick, 2 to 3 inch layer of mulch will prevent the soil from drying out between periods of rain or irrigation. That steady moisture helps plants to absorb calcium consistently.

Another way that plants have trouble absorbing calcium is if their roots are damaged. This often happens when gardeners cultivate around their plants to remove weeds. Preventing weeds from growing in the first place is a better strategy. That thick layer of mulch that keeps your soil moist will also prevent weed seeds from germinating thus eliminating the need to cultivate around your plants and possibly damaging the roots.


Blossom end rot is a common problem, especially in tomatoes. Prevent it from happening in your garden by properly amending your soil to ensure the correct amount of calcium and making sure that your garden is watered consistently throughout the growing season.

© 2018 Caren White

Comments

Caren White (author) on September 04, 2018:

Glad you enjoyed it. Thanks for reading and commenting.

Eman Abdallah Kamel from Egypt on September 04, 2018:

A very informative article and helpful. thanks for sharing.