Blossom end rot is a particularly frustrating problem with tomato cultivation. By the time it shows up in your maturing fruit, it’s too late to do anything about it, either for that fruit or for any others that are vulnerable to it.
This is because blossom end rot, or BER as it’s commonly referred to, is usually caused by a nutritional deficiency, and with a few exceptions, it’s very hard to rapidly change the nutritional status of your plants. The notable exception is if you’re growing hyrdoponically, but that’s not included in the scope of this article. Unfortunately, fruit with BER should be destroyed.
Causes of Blossom End Rot (BER)
There are a number of reasons your plants might suffer from blossom end rot, including:
1- Calcium deficiency.
This is by far the most common cause of BER. Tomatoes use calcium to build their skins, so a calcium deficiency results in thin and weakened skin and the characteristic black, soft spot at the blossom end of the tomato
2- Water stress.
Water stress typically combined with calcium deficiency. Because they are a soft-skinned fruit, tomatoes are particularly vulnerable to water stress, which can include lack of watering or uneven watering (long periods of drought followed by excessive amounts of water).
Ideally, tomatoes should receive a regular and ample supply of water—in hot weather, a large tomato plant can transpire an unbelievable amount of water in a single day.
3- Thinning of tomatoes.
This is a somewhat less common cause among home growers, who tend not to thinning their fruit as aggressively as commercial growers, but if you’re sure your calcium levels are adequate and you’re watering correctly, this could be the issue.
Thinning tomatoes aggressively results in higher levels of fruiting hormone being directed to each fruit, which can stress newly developing fruit and lead to BER.
4- A high-ammonium fertilizer
Using a fertilizer with a high nitrogen level derived from ammonia can sometimes cause BER because it competes with calcium for uptake into the plant’s roots.
Ultimately, BER is a little bit heartbreaking because by the time you see it, it’s too late to save that fruit and any others that are going to be affected. The best option is to try to correct any problems. First, make sure you’re watering evenly and adequately.
This will encourage a steady movement of calcium through the plant and to the fruit. Second, if you haven’t already, switch to a liquid fertilizer, which is more immediately available to the plant, with a high calcium level. And next season, make sure your growing medium, whether a container-based potting mix or the ground, is adequately prepared with high-calcium supplements.