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What is blossom-end rot and what can be done about it?

In the Garden

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It’s kind of discouraging to see what looks like a nice tomato, only to pick it and find a dark, leathery patch on the bottom.

While blossom-end rot is most often seen in tomatoes, it will also affect peppers, melons, squash and eggplant, although to a lesser degree.

Blossom-end rot is caused by a calcium deficiency at the blossom end of the fruit. This is strange, since the base mineral of eastern Idaho soils is calcium. Areas that have a low pH may experience an actual calcium deficiency. In these cases, lime (calcium carbonate) is added to the soil. Eastern Idaho doesn’t need that.

So, if we have plenty of calcium in our soil why do we get blossom-end rot?

The main reason is because of inconsistent watering. Calcium only moves through the xylem tissue, which carries water and minerals from the roots to the upper plant. The other type of vascular tissue is the phloem, which carries the food produced in the leaves to the rest of the plant. Calcium does not move through the phloem tissue.

When the plant gets irrigated after being slightly or severely drought stressed, the water solution from the roots goes to the site of photosynthesis and transpiration first (the leaves), taking the calcium with it, and not so much to the fruit. This foliar calcium cannot move from the leaves to the fruit, so the fruit ends up with a calcium deficiency. The calcium deficiency results in abnormal cell wall development. A water spot develops and over time it dries and forms a dark spot. Sometimes secondary infection organisms enter and rot the fruit.

Any practice that interrupts the constant flow of water to all parts of the plant can result in calcium deficiency in the fruit. For example:

  • Inconsistent watering
  • Overwatering soils promote poor root development.
  • Shallow daily watering prevents deep roots from developing.
  • Tilling too close to the stem may damage the roots, reducing water/calcium absorption.
  • Over-fertilization or too much nitrogen fertilizer promotes rapid vegetative growth, redirecting the calcium away from the fruits.
  • Potting soils, used in pots or raised beds may not have enough calcium to meet the plant’s needs.

Adjust your management practices to reduce the incidence of blossom-end rot.

  • Promote deep roots when the plants are small by deep, infrequent watering.
  • Once fruit starts to develop keep soil moisture as even as possible without waterlogging.
  • Use mulch to help conserve soil moisture, mulches should be left on the soil surface, not tilled in.
  • Weed control activities, such as hoeing or tillage, should be shallow and not close to the main stem.
  • Any fertilizer applied while fruit is developing should be low in nitrogen and higher in phosphorus.
  • Acidic soils and potting soils should have a handful of ground limestone added and mixed in before planting transplants.

Foliar sprays are ineffective. The calcium that lands on the fruit can be absorbed and used, but the calcium on the leaves will not reach the fruit.

Some cultivars are more susceptible to blossom-end rot. If blossom-end rot is an annual issue, consider planting different cultivars.

Finally, the fruit that has the dry blossom-end rot patch is still edible, but it may not taste as good as you want.

For questions on gardening topics, you can reach Ron at (208) 529-1390.