What is going wrong

[lfcjohn]

I am a little worried about one of my tomato plants (COSTOLUTO FIORENTINO) the fruits are forming well but then are starting to go black underneath as the photo shows. Any suggestions about what is causing it and how to cure it?
Many Thanks
John

[supergrass]

A. Blossom end rot, caused by improper (fluctuating from too dry to too moist) moisture. Maintain uniform soil moisture as the fruit nears maturity. Remove affected fruit.

Max.

[Tam]

Supergrass is right the most common reason for blossom end rot is irregular watering. Lack of calcium can also be a cause.

[Two_Sheds]

It is actually lack of calcium, caused by poor watering technique. The roots dry out and the plant can't get enough calcium.

You need to remove the affected fruit, correct your watering technique (constantly moist compost is ideal), and you should be okay.

[Cutecumber]

It can be cause by over-watering, too... just complicate matters

[pjh75]

I had this... someone on here (sorry, can't remember who) recommended adding lime to the soil which I did and all seems okay since.

[SarzWix]

Incidentally, you can leave the fruits on the plant to mature, as long as the black patches don't look mouldy. You just cut the black bit off the bottom & eat the rest