Mango tree disease

Gunth starts with ...
Recently bought this mango tree from bunnings at the start of summer, its grown well and is around 6ft now.
Lately the leaves have been falling off and iv noticed black spots all over them.. starting on the lower leaves and slowly spreading up the tree.
Iv been told to sprayer it all over and under with copper spray and remove all the spotted leaves, upon doing so iv noticed that the issue still remains unsolved.
As of today there is ~60% less leaves and im worried im going to lose them all, the remaining leaves on the tree are almost unmarked .. some have a pin head sized black dot on the underside of the leaves.
what should i do from here

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