How to Identify and Prevent Blossom-End Rot

Photo Credit
Chris Burnett
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What To Do When You See Blossom-End Rot

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Do your tomatoes look like they are rotting? It is probably blossom-end rot! This frustrating problem can seemingly come out of nowhere. Here are top tips for preventing blossom-end rot on your tomatoes and other garden plants.

What Is Blossom-End Rot?

Are your tomatoes developing a dark, sunken spot on their bottoms? Unfortunately, they likely have blossom-end rot. It’s a common problem for fruiting garden plants such as tomatoes, pepperseggplants, and squash. Blossom-end rot is a disorder in which the tissue of the blossom end (the “bottom”) of the fruit breaks down and rots, thus reducing the quality of the fruit and the overall yield from the plant.

What Causes Blossom-End Rot?

Thankfully, blossom-end rot isn’t caused by a disease or pest. Instead, it is the result of a lack of calcium in the plant. This lack of calcium may be due to low calcium levels in the soil or—more often—soil that is over- or underwatered. Wide fluctuations in soil moisture reduce the plant’s ability to take up calcium from the soil. When the demand for calcium exceeds the supply, the tissues in the fruit break down, and blossom-end rot occurs.

In addition to watering issues, calcium deficiency during fruit formation may also be caused by:

  • Too much nitrogen-heavy fertilizer
  • Improper soil pH
  • High salt levels in the soil
  • Damage to plants’ roots

Awareness of these causes when caring for tomatoes and other garden plants is the best way to prevent blossom-end rot.

Some blossom-end rot is more or less normal in the first tomatoes of the season since the plants are usually under stress during the initial fruit set. It also tends to happen more often in plants grown in containers, as the soil is more susceptible to fluctuations in moisture. In other words, the plants dry out too much, and their fruit is affected. 

If the damaged portion of the fruit is small, you can trim it off and enjoy the rest of the fruit. Hopefully, the problem will fade with the prevention tips below!

Blossom-end rot on tomatoes. Photo by Chris Burnett
Blossom-end rot starts out as a small, bruised area near the bottom of the fruit. Photo credit: Chris Burnett

Signs of Blossom-End Rot

Usually, blossom-end rot appears while the fruit is still green or ripening, so it often affects the first fruits formed on the plants.   

Blossom-end rot starts with a small, depressed, water-soaked area on the blossom end of the fruit (the bottom, opposite the stem). The spot starts off looking like a dark bruise. As the spot enlarges, it becomes sunken and turns black or dark leathery brown in color. Half the fruit may eventually be affected.

blossom end rot on tomatoes
Blossom-end rot appears as brown, sunken spots on the bottom of the fruit. Photo Credit: North Carolina State University.
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How to Prevent Blossom-End Rot

As mentioned above, blossom-end rot is a condition caused by a lack of calcium. Unfortunately, the rot will not go away once a fruit is already affected, but following the advice below can stop the rot from progressing further and from affecting other developing fruit.

  • First, choose vegetable cultivars that are tolerant of calcium deficiencies and less likely to show blossom-end rot symptoms.
  • Inconsistent watering is the main cause of calcium deficiency. Avoid watering your plants too much or too little. Instead, water consistently and evenly. If you forget to water, do not overwater. If it’s rainy, ensure plants have good drainage and the soil dries out (but do not cease to water completely). Overall, plants need about 1 inch of water (about 0.6 gallons) per square foot per week.
  • Use mulches to retain soil moisture during dry periods.  
  • Have your soil tested periodically to determine if there is sufficient calcium in the soil. If you’re using new, store-bought soil, it will likely have more than enough calcium in it already. However, if you’re growing in older soil, calcium levels may be depleted. If your soil test confirms this, add an organic source of calcium such as lime, bonemeal, or finely crushed eggshells. 
  • Check the soil pH on a regular basis, particularly if you use lime as a calcium source. A pH of about 6.5 (slightly acidic) is ideal for growing most vegetables, as it allows for the best nutrient uptake.  
  • Because blossom-end rot is caused by a calcium deficiency, you might be tempted to just fertilize your plants more. However, in most cases, there is already sufficient calcium in the soil; the plants just cannot absorb enough of it. Overfertilization is also harmful, and having too much nitrogen, magnesium, and potassium in the soil can actually make blossom-end rot worse. If you are going to fertilize, follow this advice:
    • Consider using a fertilizer that is formulated to contain more calcium.
    • Use a product that contains nitrate nitrogen instead of ammonium nitrogen (as the latter may contribute to more blossom-end rot).
    • Avoid over-fertilizing during early fruiting, when blossom-end rot is more likely to occur.
  • Avoid cultivating or digging near the roots of plants, as root damage can affect their ability to absorb water and nutrients.
    • Staking plants like tomatoes while still young and more adaptable is also a good idea. See our video on properly staking tomatoes.
About The Author

Catherine Boeckmann

Catherine Boeckmann loves nature, stargazing, and gardening so it’s not surprising that she and The Old Farmer’s Almanac found each other. She leads digital content for the Almanac website, and is also a certified master gardener in the state of Indiana. Read More from Catherine Boeckmann

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