Blossom end rot

Blossom end rot is a dark, leathery, sunken patch that develops on the blossom end, the bottom, of tomatoes, peppers and some squash, rather than a disease caused by a pest or pathogen. It is a physiological problem linked to the plant not moving enough calcium into the developing fruit, which in containers is usually driven by uneven watering rather than a true lack of calcium in the soil.

In balcony containers, this uneven watering is often the result of pots drying out fully between waterings, especially during hot or windy stretches, which interrupts the steady water flow the plant needs to carry calcium up into fast-growing fruit. Overly small pots make this worse, since less soil volume means moisture levels swing faster between a thorough watering and the next dry spell.

Preventing it comes down to consistent watering rather than any special fertilizer, keeping the potting mix evenly moist rather than alternating between soaked and bone dry, along with using a large enough container and a layer of mulch on top to slow moisture loss. A common mistake is reaching for a calcium supplement first, when fixing the watering routine and container size usually resolves the problem within a few fruiting cycles.