This is one of a series of limericks Asimov wrote about various poisons in 1984. (Amanita phalloides is the death cap mushroom.) Like the other limericks in this series, it appeared in the “Detectiverse” feature of Ellery Queen’s Mystery Magazine (the May 1984 issue, in this case). It has all the virtues and vices of Asimov’s limericks generally: it’s clever but not really rib-tickling. At least it scans without problems.