Apple leaf disease??

I only got this wee apple sapling 2 wks ago - bought as "pre-bonsai" & looked very healthy in the photo prior to buying. Originally in a tiny bonsai pot, I just repotted it into a slightly larger one a few days back. But pretty much from the minute I got it, it started developing brown areas on the leaves which I know can be down to watering issues sometimes? It's currently in a 1/2-1/2 peat/inorganic mix but things just seem to be worsening. Is it recoverable?