What is the likely cause for leaves turning white?

I can't name the plant affected, but it's a small tree which I'm informed
will climb against a fence. I was given it about a month ago and have
planted it in a pot with potting compost. Two leaves have turned white on
the upper face, the underside still being green.

What could be the cause of this, and what should I do in an attempt to fix
it?

Ta.

John