Blossom end rot is a troubling disorder of tomatoes that affects home gardeners and commercial growers alike. Numerous foliar calcium sprays are available that claim to solve this problem, but are they really the answer?
In addition to tomatoes, blossom end rot can affect peppers, watermelon, eggplant, squash, and cucumbers. Blossom end rot is the result of a localized calcium deficiency in the fruit; however, this may not necessarily indicate a calcium deficiency in the soil or in other parts of the plant. Blossom end rot can occur even when calcium levels in the soil are adequate.
Calcium is highly mobile in plants, and a consistent supply is needed during fruit set for the fruit to develop properly. Calcium is taken up from the soil through the plants’ roots and is transported by the xylem, along with water moving up and into the growing points of the plant. This requires an adequate supply of water in the soil. If the supply of water taken up by the roots is interrupted, the supply of calcium is also interrupted.
Calcium that is present in leaves is not able to move through the phloem into the fruit. In addition, the skin of the fruit is not able to directly absorb foliar-applied calcium. As a result, foliar calcium sprays are not effective.
The most effective way to prevent blossom end rot is to maintain uniform soil moisture. In other words, avoid allowing the soil to dry out in between watering. Setting up a drip irrigation system on a timer is a great way to ensure consistent watering. To reduce moisture evaporation from the soil, apply a layer of mulch or pine straw around the plants. Also, avoid damaging roots by hoeing or tilling near established plants, as doing so may reduce their ability to take up water. Take a soil test well before planting and follow the lime and fertilizer recommendations to ensure appropriate levels of calcium are present.
For more information on blossom end rot, see HGIC 2217, Tomato Diseases and Disorders.