What Is Blossom-End Rot?

Blossom-end rot can be mistaken for a plant parasite or virus. It is usually caused by inadequate water and calcium in the soil.
Blossom-end Rot

Blossom-end Rot

Vegetables planted while the soil is still cold are likely to develop blossom end rot on the first fruits; planting in warmer soils helps to alleviate the problem.

Photo by: Image courtesy of Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Image courtesy of Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Vegetables planted while the soil is still cold are likely to develop blossom end rot on the first fruits; planting in warmer soils helps to alleviate the problem.

Blossom-end rot is a disturbing disorder affecting fruiting vegetables like tomatoes, squash, eggplants and peppers. It first resembles a small water-soaked spot at the blossom end of fruit. Blossom-end rot spreads quickly forming a black, leathery spot or crater-like indentation and can cover half the fruit. Secondary diseases or mold can also form on the affected areas, overtaking the entire fruit. This destructive malady is most common in the earliest fruit of the season, but if left unaddressed, will continue throughout the growing season and crop losses can reach 50% or more.

The development of blossom-end rot can be mistaken for a parasitic attack or viral disease. But blossom end rot is actually a physiological disorder caused by environmental conditions. Calcium deficiency and inappropriate moisture are the primary causes. Blossom-end rot cannot be transmitted between plants or by contact, but is likely to affect all fruit-bearing plants within a planting site.

When blossom-end rot becomes a problem, quick fixes are challenging. Because it is not viral and not a result of pest infestation, chemical treatments like fungicides or insecticides are of no value.

To prevent blossom-end rot, site preparation is essential. Establishing a soil pH around 6.5 allows for necessary calcium uptake. Amending the soil using lime or composted manure will help reach optimum calcium levels, but is slow to reach efficacy and must be applied before planting to have a strong impact. When preparing the soil, avoid overusing ammonia nitrogens, which can inhibit calcium distribution. Nitrate nitrogen and fertilizers rich in phosphorous are preferred.

Unstable moisture conditions are also a component in the development of blossom-end rot. Establish a stable watering routine, avoiding drought stress risk, but also taking care not to overwater crops. Irrigation is especially effective and mulching can help stabilize moisture conditions when drought conditions are a concern.

Once blossom-end rot has set in, steps can be taken to minimize crop impact, but turnaround is slow. Problems with moisture delivery must be resolved as quickly as possible, changing watering strategies as needed and applying mulch to unprotected soil. Calcium deficiencies must be diagnosed and corrected. Calcium chloride solution sprays may be considered, although plant absorption and movement to affected fruit through foliar applications of calcium suggest impact will be minimal and can be phytotoxic in excessive amounts. Although it is not a “quick fix,” calcium soil amendments are the most reliable method for resolving calcium deficiency issues.

When it comes to blossom-end rot, the best offense will always be a good defense. In addition to establishing correct soil and watering conditions, delaying early planting of blossom-end rot-prone crops can help to avoid onset. Blossom-end rot is most common in early crops. The likelihood of blossom-end rot is increased in plants that have been committed to the soil in cold weather and can be avoided by waiting until the soil is warmer. A delay in establishing fruit-bearing crops that are prone to blossom-end rot by just a week or two can lessen blossom-end rot risk considerably, avoiding early season crop losses to this vexing disorder.