How to Prevent Blossom-End Rot, a Common Tomato Plant Disease That Can't Be Reversed

Tomatoes are one of the most versatile and popular plants for backyard gardens, but they can be affected by a variety of unwelcome diseases, including blossom-end rot—which is inedible and irreversible. Knowing how to recognize and prevent blossom-end rot is key to keeping your tomatoes healthy, so you can keep your summer menu filled with these juicy fruits all summer long. To help you protect your crop from this disease, we spoke to gardening experts who shared their tips for preventing blossom-end rot.

What Is Blossom-End Rot?

Blossom-end rot is a disease that causes the ends of a plant's fruit to develop areas of black rot. Though blossom-end rot can affect the fruits of several different plants—including peppers, squash, pumpkins, eggplant, and watermelons—it's especially common in tomatoes.

Blossom-end rot can show up on any type of tomato, but you're more likely to see it in larger varieties, like paste and plum, than in smaller ones, like cherry or grape. The disease is so frequent that you shouldn't see it as a failure of your gardening skills. "It's the very lucky tomato gardener who never sees this problem," says Jon Traunfeld, extension specialist and director at the University of Maryland Extension home and garden information center.

Signs of Blossom-End Rot

Signs of blossom-end rot include sunken, leathery, or rotted areas near the blossom end of the fruit. "It appears when the fruit is still developing or has reached a certain size," says Carrie Spoonemore, co-creator of Park Seed's From Seed to Spoon app. "It will begin to appear as a small, water-soaked spot and can turn into a large, black, rotten spot."

While tomatoes are the victims of several common diseases, none of the others produce rotten spots that are the same color and in the same spot as blossom-end rot. Don't mistake blossom-end rot for anthracnose (a fungal disease that affects the sides and tops of tomatoes), late blight (brown lesions on the fruits, leaves, and stems), or sunscald (bleaching that turns the sides of tomatoes white or yellow).

The Most Common Cause of Blossom-End Rot

Blossom-end rot is caused by low calcium levels within the fruit that destabilize the cell walls, allowing them to break down. "Calcium is much less mobile in the plant vascular system than other nutrients," says Traunfeld. "When insufficient calcium is taken up by plant roots, it goes to the growing points—new leaves and shoots—and reaches the bottoms of developing fruits last." This often happens in early or mid-summer, during the plant's fastest growth period, when calcium levels can't keep up with the plant's requirements.

How to Prevent Blossom-End Rot

Blossom-end rot can't be stopped, but you can prevent it on future fruits by checking the calcium of your soil and tending to environmental stressors that can compound the problem—like low soil moisture, soil compaction, root injury, lush plant growth, hot weather, and high light intensity, says Traunfeld.

Maintain Calcium Levels

Calcium deficiency is the main cause of blossom-end rot in tomatoes, so it's important to ensure sufficient calcium is available. Start by testing your soil to determine its calcium content. "If the test indicates low calcium levels, you can enhance the soil by incorporating calcium-rich materials like gypsum or crushed eggshells," says Spoonemore. "Calcium-based foliar sprays, applied directly to the plants leaves, can also help increase calcium levels."

Water Consistently

One way to prevent blossom-end rot is by maintaining consistent moisture levels in the soil. "Avoid over and underwatering," says Spoonemore. "Watering consistently helps to ensure that the calcium in the soil is available for uptake by the roots." When overwatering occurs, the calcium is flushed out of the fruit more quickly, which causes a deficiency.

Apply Mulch

Another way to maintain consistent moisture levels and head off blossom-end rot is with mulch, which keeps the soil moist but not waterlogged. "Apply a layer of organic mulch around the base of the plant to maintain stable temperatures and moisture," says Spoonemore.

Maintain pH Levels

Maintaining the correct soil pH level is essential for preventing blossom-end rot. "The ideal pH range for most plants is slightly acidic to neutral—around 6 to 7," says Spoonemore. "Test the pH of your soil using a soil testing meter, and if necessary, adjust it by adding materials to bring it into the appropriate range."

Avoid Excessive Digging

Too much digging around the plant's root system can lead to water stress and calcium uptake issues. "It is important to avoid unnecessary digging around the plants, especially when the fruits are forming and to maintain a healthy uptake of nutrients, including calcium," says Spoonemore.