You can buy a plastic welder that heats the plastic and feeds a plastic rod into the crack. it works better on some plastic than others.
You could also drill 3/16" holes about 1" apart and right across from each other along the crack and use zip ties to hold it together. This is what I call Frankensteining it and actually is quite strong.
The best repair is plastic welding.. Slice off a few shards of plastic from an inconspicuous area, so your filler material is the same as the fender. A 10 dollar HF heat gun will do the trick for that.
You can also screw sheet metal underneath the plastic, which is what I had to to do to keep the fender from falling off our old 6x4 Gator.
If I didn't have a plastic welder that ohiosteve mentioned, I would put a plate on the underside like Atimm said... The plastic welder I have has fiber reinforced filler rod (black) that is darned strong.. I've fixed a lot of chain saw plastics with it and it holds up well.. the welder itself is rather expensive, but you could do with a propane powered soldering torch or something similar to do the melting