On Drum Island, after Chopper joined Mugiwara's crew, and they left, Dr. Kureha fired sakura powder (I forgot the name of this powder) that Dr. Hiluluk created as a farewell present for Chopper.
At that time, there's some conversation between Dr. Kureha and Dalton. Dr. Kureha asked Dalton if he knew about Gol D. Roger.
It seems that Dr. Kureha knew Gol D. Roger, but how did she know about him? What is the relation between Dr. Kureha and Gol D. Roger?
I don't remember if this is ever explained in the anime or manga.