I'm wondering the ruiling on Bestow Curse and concentration.

Let's say my cleric already has "Bless" active and concentrating on that. 

Then he casts "Bestow Curse" with a level 3 spellslot on an enemy, but the enemy SAVES against the spell...

Does he still keep his concentration on "Bless"?

Casting a concentration spell immediately ends any already active concentration spell. From sage advice compendium

If I’m concentrating on a spell and I cast another spell that requires concentration, when does my concentration on the first spell end?

If you’re concentrating on a spell, your concentration on it ends immediately when you start casting another spell that requires concentration.

 Thank you! That was a fast answer!