Most growers are familiar with blossom-end rot in tomatoes, but it can look very different from fruit to fruit. The most unusual is probably internal blossom-end rot. This shows no external symptoms (or very subtle symptoms), but there is a large black area inside the fruit. In peppers, blossom-end rot often shows up on the side of the fruit, rather than the end. Below are some photos of blossom-end rot (BER) in tomatoes and peppers.
Although blossom-end rot is an extremely common problem, which has been studied for over 100 years, the exact causes – or how to reliably prevent the problem – is not yet understood. Most sources explain it as triggered by a temporary lack of calcium in the blossom-end of the fruit as it is developing. Since calcium moves with water in the plant, this is usually thought to be caused by interruptions in the water supply to the fruit and not so much by an overall calcium deficiency. However, many other factors seem to influence the problem as well. In fact, there is now some thought among scientists that low calcium levels in the fruit are not the trigger for blossom-end rot at all.
The list of other factors that may influence blossom-end rot is a long one.
- stress factors, such as dry conditions, that reduce fruit growth
- high temperatures and intense sunlight, especially following cooler, overcast weather
- high ammonium-nitrogen levels in the soil
- high salinity
- susceptible varieties
- stress occurring during periods of rapid fruit growth
- potassium/calcium ratios in the fruit
- high nitrogen fertilization
- fluctuations in levels of various growth hormones in the plant
There is now speculation that stress-free, rapid growth conditions create the susceptibility for blossom-end rot in fruit at a certain stage of development. If conditions suddenly turn stressful, reducing growth rates, blossom-end rot is thought likely to develop. Unfortunately, this does not provide us with a recipe for preventing the problem, so researchers continue to study the disorder.
As mentioned, variety susceptibility in tomato is a major factor. Those varieties with the highest level of soluble solids are usually the most susceptible. The other factors that result in blossom end rot also increase soluble solids ie. drought. High soluble solids varieties can be prevented from getting blossom end rot by judicious irrigation but this will reduce the soluble solids. You can see why breeding higher soluble solids hybrids can be a challenge.