When the fender came off, so did all of that mess of leaves and twigs that were trapped by the lower part of the fender. The fender is not rusted, but dirty in that area in this picture (luckily). Water drains from the firewall cowl down thru this area. Also any "junk" that makes it's way past the wipers and onto the firewall cowl. Best clean that stuff off every chance you get or it will end up trapped by the fender, there to rot and rust.