Tomatoes – Blossom end rot


Blossom end rot. Tomatoes are grown in pots in triple mix soil.Kept watered and fertilized every 3 days with 20 /20/20. They get lots of sun on a patio just north of Toronto in Alliston zone 4.


Sorry to hear about this! Blossom end rot is not due to disease or pests – but rather to adverse growing conditions.  It often occurs where fruit develops rapidly when it’s hot and dry – and we’ve had lots of that type of weather this summer (2016).

Blossom end rot is mainly due to calcium deficiency in the soil, which – in the fruit itself – disrupts the permeability of the cell membrane, resulting in cell swelling, leakage and ultimately breakdown of membrane. As well, fewer new cells grow. This results in black, sunken areas of the fruit, which is vulnerable to attack by bacteria, fungi and pests.

Fluctuations in the moisture available to the plant are likely responsible for the calcium deficiency. This is often an issue in plants grown in pots, where irregular water supply occurs quite frequently.

To combat blossom end rot, it’s all about ensuring that the plant has a consistent source of water:

  • Ensure soil/potting mix is not allowed to dry out between waterings – this means that the plant’s roots may also dry out – during a hot summer like this one, plants may need watering at least twice a day.
  • Remove the affected tomatoes immediately, to give other fruits the opportunity to develop normally.
  • Conserve moisture around plant roots by covering potting mix with mulch.
  • Apply fertilizer to moist soil – not to dry soil, as the concentrated nutrients can restrict the plant’s water uptake.

A common misconception is that because the plant is lacking calcium, its foliage should be sprayed with calcium salts.  This is not helpful, as the calcium won’t be absorbed by the tomato fruit and even if it is absorbed by the leaves, most of it won’t be transported to the fruit.  What is needed is consistent watering, so that the plant’s root system can absorb and deliver sufficient calcium to the rest of the plant.

Here are a couple of good resources:

Missouri Botanical garden. Tomato diseases and disorders.

Toronto Master Gardeners. Calcium for tomato blossom end rot.