Inhibitors of deoxynivalenol production by Fusarium graminearum are useful for protecting crops from deoxynivalenol contamination. We isolated precocenes and piperitone from the essential oils of Matricaria recutita and Eucalyptus dives, respectively, as specific inhibitors of the production of 3-acetyldeoxynivalenol, a biosynthetic precursor of deoxynivalenol. Precocenes I and I I and piperitone inhibited 3-acetyldeoxynivalenol production by F graminearum in a liquid culture with IC(50) values of 16.6, 1.2, and 306 mu M, respectively, without inhibiting fungal growth. Precocene 11 also inhibited deoxynivalenol production by the fungus in a solid culture on rice with an IC50 value of 2.0 ppm. Precocene 11 and piperitone decreased the mRNA levels of Tri4, Tri5, Tri6, and Tri10 encoding proteins required for deoxynivalenol biosynthesis.