Location: north Italy (Alps). Asking for my dad.
My dad has planted some 20 tomatoes of various type in his garden, in a spot with around 8h or sunlight and automatic watering through a drip-tube at the base. He sometimes alternates by planting potatoes, and in spring before planting he usually turns the earth around and adds some new/fresh earth/compost. He has been doing this for years and always has good harvest.
This year he noticed an issue on some of the plants: the leave starts to dry up and wither, and some fruits starts to have some spots as if they are starting to rot. So far it seems as only the regular round tomatoes are affected by this, but it seems to be spreading.
Reading up online about it could be overwatering, but we doubt it is that. We read it could be some apical rot caused by lack of calcium, but we are not sure.
What is causing this? Can the plants still be saved? How can this be prevented next year?