There is a way, and it might already be somewhat of a trope but you can use fruitpits to extract amygdalin which can be then refined with a homelab to a cyanide like substance. Although not pure or concentrated enough, but it could be added to food by a wife on a very long period of time and make the Victim very I’ll with fatal results.

Amygdalin is extracted from almonds or apricot kernels by boiling in ethanol; on evaporation of the solution and the addition of diethyl ether, amygdalin is precipitated as white minute crystals.