The addition of autoclaved mycelia of Aspergillus niger and the known phytopathogenic fungus Phytophtora cinnamomi to cultured cells of Cinchona ledgeriana Moens. caused a marked increase in the anthraquinone content of the plant cells. This finding in combination with the antimicrobial activity of the anthraquinones isolated from calli of Cinchona pubescens Vahl. led to the conclusion that anthraquinones are phytoalexins.

The contents of secondary metabolites (alkaloids and anthraquinones) in Cinchona ledgeriana bark infected with Phytophthora cinnamomi and in non-infected bark were determined. It was found that the alkaloid content of the infected bark was lower. Whereas no anthraquinones could be detected in the healthy bark, these compounds were present in the infected bark. So, anthraquinones in Cinchona act as phytoalexins.