Blossom end rot

Blossom end rot is a physiological problem, caused by adverse growing conditions rather than a pest or disease. Certain vegetables that form large fruits, such as aubergines, peppers and (most often) tomatoes are particularly susceptible.

Blossom end rot shows as a circular patch, varying in colour from greenish brown to black, at the end of the fruit that had the flower (the end furthest from the plant). As this patch increases in size it becomes sunken so that the fruit has a flattened appearance at the affected end. The blackened patch varies greatly: in some fruits it is only 1cm (1/2in) across, while in others it is 2.5cm (1in) or more in diameter.

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

To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key nutrients usually taken up from the soil are nitrogen, phosphorus and potassium. Plants also need carbon, oxygen and hydrogen to make carbohydrates — their main energy source. Carbon and oxygen come from the air as carbon dioxide, while hydrogen is drawn from water absorbed by the roots. Other important soil nutrients include magnesium, calcium and sulphur. Most garden soils contain enough of these nutrients to support healthy plant growth. 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

The concentration of water vapour in the atmosphere. This affects the rate at which plants transpire (lose water from their leaf pores), which in turn affects how much water their roots absorb from the soil. If air humidity is low, plants transpire quickly and become at risk of wilting if water is lost from the leaves faster than it can be absorbed by the roots. Different plants require different levels of humidity to thrive. Many tropical houseplants need high humidity and are best grown in a steamy bathroom, misted regularly or placed on a saucer of damp pebbles. In a greenhouse, humidity can be raised in hot weather by damping down (wetting) the floor or overhead misting. can also limit water uptake by plant roots, so good daytime ventilation

The introduction of outdoor air into a space, for example, a greenhouse. This airflow can reduce temperature, replenish the gases absorbed by plants and keep down fungal diseases like grey moulds. 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
  Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own. should be kept consistently moist throughout the growing season
  The period of time when an individual plant is in active growth. This will depend on the local climate and light levels, and can vary between different plants, although it is broadly from spring to autumn. 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
  Mulch is a layer of material, at least 5cm (2in) thick, applied to the soil surface in late autumn to late winter (Nov-Feb). It is used to provide frost protection, improve plant growth by adding nutrients or increasing organic matter content, reducing water loss from the soil, for decorative purposes and suppressing weeds. Examples include well-rotted garden compost and manure, chipped bark, gravel, grit and slate chippings. 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