I bought a big bag of very large russet potatoes.  I baked five of them and when I cut them open there were large streaks of black stuff through all of them (see photo).  A week prior to baking these, I used some other potatoes from this same bag to make a casserole using raw potato slices; after peeling and slicing the potatoes there was no evidence of this.  These baked potatoes were only a week older than the potatoes I sliced which were fine.  I have never seen this before in a potato.  What is it?