Blossom end rot is caused by lack of calcium in the fruits. Calcium deficiency reduces cell membrane permeability and this leads to swelling of the cells followed by leakage and destruction of the membrane structure. There is also a reduction in growth of new cells. This causes the characteristic dark, sunken areas.

It is very rare for soils, growing bags or potting media to actually lack calcium. There is almost always plenty in the soil and indeed within the plant. However for calcium to reach the parts of the plant that are furthest from the roots there needs to be a good flow of water through the plant. The fruits are distant from the roots and do not compete as well as the leaves for calcium, leading to a local lack of calcium at the ends of the fruit.

Plants grown with limited root space (e.g. in pots or growing bags) are most at risk of irregular water supply leading to local calcium deficiency in the fruits. Plants in border soil, whether outdoors or in the greenhouse, are less likely to be affected.

A similar problem can arise if fertiliser is added to dry soil around plants, because the concentrated nutrients in the soil water will restrict water uptake by the plant. Additionally, some fertiliser ingredients - ammonium salts for example - compete with calcium for access to the plant roots, further exacerbating the calcium deficiency.

Very high air humidity can also limit water uptake by plant roots, so good daytime ventilation of greenhouses is helpful.

There is nothing you can do to save fruits once they have blossom end rot, but you can prevent subsequent fruits from being affected by the following:

• The soil or potting compost should be kept consistently moist throughout the growing season and must never be allowed to dry out. During hot periods it may need watering two or more times a day. It is better to water twice a day than once with a double volume. Growing bag compost is very difficult to moisten evenly and thoroughly once it has dried out
• Automatic watering systems, use of larger containers, or growing in border soil are all potential solutions where frequent watering is not possible. Covering the soil or potting compost with mulch will also help to conserve moisture around plant roots
• Applying liquid fertiliser may help, but this must only be applied to moist soil, and following the manufacturer's recommendations
• Applying foliar sprays of calcium salts such as calcium nitrate are not effective because calcium is not readily absorbed by the fruits and transport from leaves to fruit is poor. The nitrate in this fertiliser can also promote lush growth that leaves less calcium available for the fruits