I know, spoilers and such. But for those who've already read chapter 676, you were probably wondering about that apple suddenly changing into a devil fruit. For those that missed that detail, shock is in order.
Speculations
Given that this is a theory, I can't say with all certainty that it is right. What I observed is that a user, the lizard-gel Smiley, died (hinges on theory and part of the dialogue, we'll see if I'm right next week). And that an apple changed into a devil fruit. My theory began way back when Blackbeard broke the rules and somehow obtained Whitebeard's Gura Gura no Mi. We were told that if a user EATS two of these fruit, he will die. However, I have reason to believe that he absorbed the powers directly in some way (that we didn't get to see).
Given the observations of both the apple and Blackbeard's dual powers, I submit the theory that on death, the devil fruit of a user seeks out a host. In this one case, it was a nearby apple. In the case of Blackbeard, it probably involved a secondary method, presumably the other unseen method used to transfer a power into an inanimate object.
Whatever the method, there has to be a way (maybe by obstructing everything else) of "marking" something for absorption.