Limited systemic protection of broad bean againstBotrytis fabaeand of cucumber againstCladosporium cucumerinumhas been observed as a result of the application of pyridine‐2‐thiol‐N‐oxide and its carboxymethyl derivative. Apparently this protection is, to some extent, the result of translocation of the chemicals in the plants. It was found that the vapour phase of pyridine‐2‐thiol‐N‐oxide plays an additional role in the chemotherapeutic activity. There is no evidence that the copper chelate of pyridine‐2‐thiol‐N‐oxide is translocatable in plants. Root applications of these three compounds had no