How to Prevent Blossom-End Rot in Tomatoes

Though blossom-end rot can affect several different fruit and vegetable plants—including pepper, squash, pumpkin, eggplant, and watermelon—it's especially common in tomatoes. Easy to identify but impossible to reverse, blossom-end rot is best tackled with appropriate soil maintenance and the right amount of watering. But even then, you may lose a few fruits to the disease. Knowing how to prevent blossom-end rot and recognize its signs and symptoms is key to protecting your tomatoes should the disease ever reach your garden beds.

What Is Blossom-End Rot?

Blossom-end rot is less common in smaller cherry and grape tomatoes, and more likely to threaten larger tomatoes, like paste and plum varieties. "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. The disease, which affects fruit-bearing plants, causes the end of the fruit to develop a black, rotten spot that makes it inedible. Once blossom-end rot infects your fruit, it is impossible to reverse.

Signs of Blossom-End Rot

Several other tomato diseases might be confused for blossom-end rot, but none have the exact same set of characteristics. "The signs of blossom-end rot are sunken, leathery, or rotted areas near the blossom end of the fruit," says Carrie Spoonemore, co-creator of Park Seed's From Seed to Spoon app. "It appears when the fruit is still developing or has reached a certain size. It will begin to appear as a small, water-soaked spot and can turn into a large, black, rotten spot."

Don't mistake blossom-end rot for anthracnose, which is a fungal disease that affects the sides and tops of tomatoes. Other common tomato plant diseases include late blight (brown lesions on the fruits, leaves, and stems) and sunscald (bleaching that turns the sides of tomatoes white or yellow).

Causes of Blossom-End Rot

At its most basic, blossom-end rot is caused by calcium deficiencies. Low calcium levels within the fruit destabilize the cell walls, allowing them to break down. This often happens in early or mid-summer when more water and nutrients are required for the plant's rapid growth. "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."

While low levels of calcium in the soil are a culprit, environmental stressors can also cause blossom-end rot. Low soil moisture from drought or irregular watering, soil compaction, root injury, lush plant growth, hot weather, and high light intensity can all contribute to blossom-end rot, says Traunfeld.

How to Prevent Blossom-End Rot

While you may not be able to reverse it once it spreads, there are several ways to prevent the disease.

Water Consistently

One way to prevent blossom-end rot is by maintaining consistent moisture levels in the soil. "Avoid over and under watering," 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, which in turn prevents blossom-end rot, is by applying mulch. Mulch 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 Excess 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.

Maintain Calcium Levels

Calcium deficiency is a main cause of blossom-end rot in tomatoes, so it's important to ensure sufficient calcium is available. To do so, 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."