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Blossom end rot...and what to do about it?

  • Wendy Irving
  • Your tomatoes ae coming along beautifully – good sized plants, nice foliage, blossoms and burgeoning fruit. Then you see something disturbing. The developing fruit is showing brown spots on the blossom end of the fruit, soon to develop into larger lesions. With the wetter than usual May this year, it is not surprising. But what is going on?

    This is unmistakably blossom end rot. Blossom end rot not only affects tomatoes, it can impact peppers, and cucurbit vegetables such as squash, eggplant, zucchini, cucumbers, and pumpkins. But, it is very common in our beloved tomatoes! Blossom end rot is a physiological disorder in which the tissue of the blossom end of the fruit (the portion of the fruit opposite the stem) breaks down and rots. Hard, discolored areas may develop inside the fruit, either with or without external symptoms. The disease is not associated with soil contact or with damage to other plant parts. So, what is it?

    Blossom end rot is caused by a lack of calcium in the fruit, but it is not as simple as it seems. Just adding calcium to the soil probably won’t do the trick. There may be low levels of calcium in the soil, but more likely there is plenty of calcium, but the ability of the tomato plant to access that calcium has been impaired in some way. Drought stress, unseasonal rain, alternating soil moisture extremes, salty and/or sandy soil, and damage to a plant’s roots all can inhibit calcium uptake, as can waterlogged or cold soils, and too much fertilizer.  Movement of calcium within plants depends on active transpiration (transpiration is the movement of water from the soil through the plant to evaporate into the air). Because leaves transpire more than fruits, calcium moves more easily into leaves, where it remains.  Calcium is not later redistributed from leaves to fruits.  This preferential distribution of calcium to leaves can be made worse by over-fertilizing with nitrogen which promotes excessive production of leaves.  In addition, high relative humidity, or low relative humidity in combination with hot, windy weather can limit transpiration, thus preventing calcium from reaching fruits. What can you do about it?

    Avoid conditions of too much or too little water.  Irrigate evenly. Do not allow the soil to either dry out or become saturated. Mulch the soil to retain moisture during dry periods.  Avoid cultivation near plants that would damage roots.  Use nitrate (NO3-) rather than ammonium (NH4+) forms of nitrogen fertilizer.  Do not over-fertilize.  Test your soil periodically to determine if there is sufficient calcium in the soil.  If not, add calcium (lime, bone meal, eggshells).  Check the soil pH on a regular basis, particularly if you use lime as a calcium source.  A pH of about 6.5 is ideal for growing most vegetables.  Finally, grow vegetable cultivars that are tolerant of calcium deficiencies and less likely to show blossom end rot symptoms.

    The good news is if you make the proper corrections now, your indeterminate tomatoes can still give you good, useable fruit as the season progresses.