It's been going on longer than the article suggests. When I went trick-or-treating in the 1960s, my parents did not allow us to eat anything that was home-made or not in a sealed package, because they were worried about deliberate poisoning.

Around 1972 or 73 on Halloween, some neighborhood kid found a razor blade in an apple. Turned out he'd inserted it himself as a prank.

A lot of things have much earlier roots than can be found with searches of documents.