Blog: How to prevent and cure this condition
Few things are as annoying for the tomato grower as spotting your ripening fruit has blossom end rot. The problem is, it can affect other large-fruiting crops such as peppers, squashes, melons and aubergines too.
This is a condition my tomatoes are often afflicted with, and it’s totally my own fault. It happens in years when I overstretch myself and have either too much work or am not in great health. I become too tired to water the tomatoes enough, and so their fate is sealed. Fortunately, plants can recover.
Fortunately, it is not a pest or disease but a condition caused by irregular watering and dryness at the roots. It’s very easy to spot and the name gives us a clue. Blossom end rot appears as a circular patch, greenish brown to black, at the end of the fruit furthest from the plant. As this patch grows, it sinks, giving fruit a flattened appearance.
The actual cause of blossom end rot is calcium deficiency, which affects the functioning of the cell membranes. New cell growth reduces, resulting in sunken patches on fruit. Read more about my trial tomatoes this year. Most susceptible varieties have larger, pointer fruit, such as plum tomatoes like San Marzano and Roma.
Blossom end rot – all down to calcium and watering
Potting compost and grow bags rarely lack calcium, and even affected plants usually have enough stores. What causes the problem? For calcium to reach the fruit, which is furthest from the roots, there needs to be a consistent flow of water through the plant. Leaves win the competition for calcium, leading to a lack of calcium at the ends of the fruit.
Plants with constricted root space are most at risk, as they dry out more quickly. Plants in border soil are less likely to be affected. A similar problem can happen if fertiliser is added to dry soil around plants. Concentrated nutrients restrict water uptake by the plant. Some fertiliser ingredients, such as ammonium salts, compete with calcium for access to the plant roots, making the problem worse.
Very high air humidity can limit water uptake, so good daytime ventilation under glass is vital. You can cut off the sunken patch and use the fruit. Prevent future attacks by doing the following:
How to save your crop from blossom end rot
- Keep the compost consistently moist and never let it dry out. This can mean watering twice a day during hot spells.
- Use automatic watering systems, larger pots, or growing in borders if you can’t water frequently. Covering the compost with mulch will help to conserve moisture.
- Applying liquid fertiliser may help, but only to moist soil.
- Foliar sprays of calcium salts don’t work. Fruits do not readily absorb calcium, and transport from leaves to fruit is poor. The nitrate promotes lush growth, which means less calcium is available for the fruits.
Blossom end rot in tomatoes, peppers and aubergines updated 2025
