Hi everyone on this forum.
I bought my first Bonsai Trees Ligustrum (Mandarin Tree) . Three threes in one pot like in pictures below.
One of them suddenly died . Maybe in 24 hours or less or leaves gone. Another 2 trees look not to bad, but have some problems as well. I think this can be some fungi ? Can anyone help me what to do?

It feels like a repotting gone wrong. If a tree drops all leaves within days of purchasing it, and others in the same set are OK, I would contact the seller.

Thank You for Your answer. I bought it in Garden Centre and I'm afraid , I don't have receipt so it will not work
Any idea what can mean these black sign on leaves?