Cynodontin: a fungal metabolite with antifungal properties.

A red pigment that accumulates in cultures of a Drechslera avenae pathotype with specificity for Avena sterilis was isolated and identified as the anthraquinone cynodontin (3-methyl-1,4,5,8-tetrahydroxyanthraquinone). Satisfactory yield of the compound was obtained with 20-60 day incubations at temperatures between 20 and 27 degrees C. Cynodontin was tested in vitro for fungitoxicity and was found to be a potent inhibitor of the growth of Sclerotinia minor, Sclerotinia sclerotiorum, and Botrytis cinerea and, to a lesser extent, of Verticillium dahliae. The ED(50) values obtained with these fungi were of the same order of magnitude as those of the commercial fungicides dicloran and carbendazim, which were used as reference chemicals. In contrast, the growth of a number of other fungi was not significantly inhibited by cynodontin. Anthraquinone and two other anthraquinone derivatives, emodin and chrysophanol, which were also included in the tests, did not affect the growth of the cynodontin-sensitive fungi. It thus appears that the type and position of the substitutions at the C-ring play a role in the expression of antifungal activity.