Blossomrot
How to prevent blossomrot
Growth disorder caused by calcium deficiency. First a grey, then a larger brown spot forms at the base of the fruit, where the tissue dries out.
Treatment
An even supply of water and nutrients can help prevent this. An oversupply of magnesium and potassium, as well as low pH levels, can also cause the problem. A quick remedy is to add a calcium-based organic fertiliser to the foliage and rock flour to the potting soil.
GreenPunkrocker, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons