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Im looking for some help! I have a fairly new and young Port Jackson Fig. I started to notice tiny rust coloured spots on one of the oldest leaves. Within 24 hours this leaf has turned quite pale and spotty, as well as these rust spot spreading to other leaves

I've tried to do some research but I just can't seem to figure out exactly what the cause could be. A fungus or improper care in some way?

Photos

Evergreen trees do shed old leafs, but not all of them at the same time.