New isolates of the blackleg fungus [Phoma lingam, perfect stage Leptosphaeria maculans (Desm.) Ces. et de Not.], virulent to the traditionally resistant brown mustard (Brassica juncea), produce L-valyl-L-tryptophan anhydride, a 2,5-dioxopiperazine different from those produced by blackleg isolates virulent to canola (B. napus and B. rapa).