I did a bit of digging after the show, he said that it would be "an explosive issue" or something- but the way he emphasized "explosive" made me think it was a hint- and so it was.

Turns out portobello mushrooms contain a decently large amount of agaritine, which can be made in to hydrazine, which can in turn be used for explosives or rocket fuel. Agaritine is also fairly cancerous/mutagenic to humans.

That's my best bet as to why he wouldn't talk about it, though the threat to his life I would think is more for comic effect.