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Prevent blossom end rot

M. Grabowski,  UMN Extension

The current warm, dry weather combined with fast growing tomato plants creates ideal conditions for blossom end rot, a common problem in garden tomatoes. Fruit affected by blossom end rot have a tan to black, flat, leathery area on the bottom (blossom end) of the fruit. 

Fungal spores may be visible on the discolored area and rot may extend into the fruit, but this problem is not caused by a pathogen.  Blossom end rot is the result of a calcium deficiency in the growing tomato fruit. Any bacteria or fungi present are secondary organisms, taking advantage of the weakened fruit.

Although blossom end rot is caused by a calcium deficiency in the fruit, it does not mean that there is a lack of calcium in the soil. Often blossom end rot occurs as a result of several cultural or environmental factors that affect the plant’s ability to take up calcium. Fluctuations in soil moisture, heavy applications of nitrogen fertilizer, and injury to roots can all predispose tomato plants to blossom end rot.

Although symptoms may not yet be visible, now is the time for gardeners to take steps to prevent blossom end rot.

  1. Keep the soil around the tomato plant evenly moist but not waterlogged.  Do not allow the soil to completely dry out between watering. Fast growing tomato plants use lots of water on a hot, sunny day. Mulch the soil with straw, wood chips, or plastic mulch to reduce moisture loss. Touch the soil to determine if it needs water. Soil type, exposure to sun and wind, and plant size will all affect how frequently a tomato plant will need to be watered. Potted tomato plants may need to be watered more than once a day. 
  2. Do not over apply fertilizer to tomato plants with young growing fruit. Excess fertilizer results in quick growth spurts that can increase blossom end rot. In addition, many of the nutrients in fertilizers compete with calcium for uptake by the roots. Tomato plants do not need fertilizer with phosphate or potash at this time, but can be fertilized with 0.15 lb/100 ft2 of nitrogen if needed. Garden fertilizers will list three numbers on the label. These numbers represent that amount of nitrogen – phosphate –potash in the fertilizer. A fertilizer containing only nitrogen will have a number followed by two zeros (16-0-0). Follow the instructions on the package to apply the appropriate amount. 
  3. Avoid injuring tomato roots. Do not dig in the soil within 1 foot of the stem. 
  4. Some varieties are more susceptible to blossom end rot. If severe blossom end rot occurs regardless of good cultural practices, consider using a different variety next year.

Do not give up on the tomato plant if blossom end rot does occur. Often the first fruit are the most severely affected. Fruit produced later in the growing season may not have blossom end rot at all.

Fruit with blossom end rot can be eaten if the rotten part is cut out and the fruit is cooked. Any type of rot can affect the natural acidity of the tomato fruit. As a result, tomato fruit with rot should never be used for canning because the natural acidity plays an important role in keeping out harmful microorganisms. The fruit can be cooked and eaten, cooked and stored, or frozen, cooked and eaten.

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