At our dog park, we have 2 mulberry trees (maybe more) and also a wild plum tree. now, every once in a while, you can find a white mulberry tree, and the one in our park is a white one. They aren't very common but they are desirable because the berries are sweeter and they don't stain anything. Katyushas parents were always WILD about the white mulberries! Toby eats the red ones that fall to the ground (and the white ones) to no ill effect other than if he gets extreme about it, it will upset his guts and he will poop purple diarrhea, but that has only happened once.
Here is a video of Katy's parents digging in the white mulberry tree. It's not very clear but i think it's a fun video.