Hi Gerry
Set your exposure compensation to underexpose the shot. With pale fungi start with something like - 1.0 (minus 1.0). Try a shot at that and check your histogram. You want to be aiming at a curve that is not cut off both sides. Referre to your manual about histograms if you don't know what they are. If that's too much at - 0.1, and the image is too dark, re-set it to something like - 0.7 and so on.
Hope this helps
Alan
edit: ^^ what dustyshadow says
^^