Hopefully Toni and Di (Georg) will notice this thread, as I know that they are quite knowledgeable in this field. I believe, however, that this is a fungal attack which can sometimes be avoided by painting the freshly felled timber with Borax. Once established, though, any fungal staining such as this can be minimised with bleach.....try painting a dark spot on the underside and leaving it for a few hours to see if it makes a difference.
Good luck....Les