Just found the source of the phrase (thanks to Skibberoo on Word Origins )
To use the phrase coined by U.S. Supreme Court justice Felix Frankfurter in a 1939 case, in these instances, the question presented is whether the challenged evidence is the “fruit of the poisonous tree.”
Dave Wilton pins down the case precisely.
The 1939 case is Nardone v. U.S., Justice Frankfurter writes, “Once that is established-as was plainly done here-the trial judge must give opportunity, however closely confined, to the accused to prove that a substantial portion of the case against him was a fruit of the poisonous tree.”