rose leaves

I take it this is an outdoor plant. If so, what's the weather been like lately? The seasons are changing (I can't recall where you are, e. g. what growing zone); it might just be a function of it being Autumn.

It could be a lot of things - fungus, virus etc. Your best chance to get an identification and treatment is to do a Google image search using the term "rose leaf diseases". Try to match up the image to your problem and see if the site posting the picture offers a solution. Most roses are tough to grow anywhere.