Blossom End Rot

Overview

Blossom-end rot, which begins with a small watery bruise on the blossom end of the fruit, is the result of a lack of calcium in developing fruits. This calcium deficiency can be a result of slow growth, damaged roots induced by extreme fluctuations in the surrounding soil moisture content, an excess of salts, or other fluctuating conditions during plant growth. The most common cause of blossom-end rot is fluctuating soil moisture. Moisture plays an important role in calcium uptake in the plant. When a dry period follows adequate moisture, calcium uptake can be reduced. Calcium deficiencies cause actively growing cells to die because they cannot retain water and nutrients. Less frequently, an actual deficiency of calcium in the soil may cause this rot. This is rarely the case in St. Louis. 

Because it is not caused by an insect or disease organism, fungicidal and insecticidal sprays are ineffective in controlling the problem. 

Symptoms and Diagnosis

Tomatoes & Peppers 

An early symptom of blossom-end rot is a light tan patch on the blossom end of the green fruit. Over time the area turns dark brown or black and may become sunken or leathery. Fruit which is one-third to one-half developed is most commonly affected. Sometimes an internal black rot will develop in the center of the fruit with little or no external symptoms. Parts of the fruit not affected by blossom-end rot may be eaten. 

Cucurbits 

Blossom-end rot begins as a water-soaked, sunken spot on the blossom end (the end that isn't attached to the stem) of the fruit. The spot may enlarge and become depressed as the fruit grows. The spot may turn from brown to black and become moldy. Blossom-end rot can be distinguished from other rots by its confinement to the blossom end. It may also be confused with rotting fruit that results from the failure of female flowers to set fruit due to a lack of male flowers or pollinating insects. 

Integrated Pest Management Strategies

1. Maintain even soil moisture. Water regularly during dry periods and mulch plants with a 3–4 inch layer of organic material to help hold in soil moisture. Be sure the soil is well-draining.  

2. Avoid deep cultivation too near plants for tomatoes. 

3. Modify your fertilizing practices. Use a fertilizer high in superphosphate and low in nitrogen. When adding nitrogen, use calcium nitrate rather than ammonia or urea forms which can increase salt levels in the soil. 

4. Get a soil test. If the above methods do not correct the problem, get a soil test and maintain proper soil pH for the plant. Calcium may not be available to plants if the soil pH is too low or high. 

5. As a last resort, use a foliar spray of calcium chloride. Do not spray too often or in excessive amounts. 

6. Plants in containers. For tomatoes, peppers, and cucurbits grown in containers, apply a fertilizer that contains micronutrients including calcium. Many fertilizers formulated for tomatoes meet these specifications. 

Organic Strategies 

Strategies 1, 2, 4, and 6 are strictly organic approaches. Organic preparations of calcium chloride (mentioned in Strategy 5) that carry the Organic Materials Review Institute (OMRI™) seal of approval are available.