The reason your not getting the desired results with this bump map, is the simplicity of it. At the moment your bump map will simply put small holes on the surface of your lemon, whereas an actual lemon has a rougher surface. With a much more lumpy surface.
Try this: (make any edits you need to it to gain the desired results.)
www.stevenegan-cgi.com
"Your weapons are no match for ours! People of Mars, surrender!"
"Um, this isn't Mars. This is Earth."
"Earth? Earth-with-nuclear-weapons Earth?"
"Yes."
[long pause] "Friend!!"