Attempts were made to determine whether Marasmius perniciosus Stahel., causal agent of the witches' broom disease of cacao, produces substances that could elicit on the host some of the symptoms which are characteristic of the disease. In vitro the fungos did not produce significant amounts of cytokinins or auxins. However, culture filtrates destroyed indole‐3‐acetic acid (IAA) rapidly because of two different IAA‐oxidizing enzymes: (1) a peroxidase (IAA oxidase) characterized by (a) dependence on Mn++ and 2,4‐dichlorophenol (DCP), (b) pH. optimum of 3.0, (c) inactivation at 85 C for 5 min, (d) stimulation of oxidative activity by addition of 0.3 μmole/ml H2O2; and (2) a laccase characterized by (a) no dependence on Mn++ and DCP, (b) pH optimum of 6.0, (c) inactivation at 95 C for 5 min, (d) ability to oxidize IAA and phenolic substrates in the absence of H2O2. The peroxidase was completely inhibited by scopoletin (0.0002 m) ; the laccase was inhibited by thiourea (0.1 m). The laccase could be separated from the peroxidase by elution through a column of Sephadex G‐100. It is suggested that the main pathogenic effect of the fungus may be associated with these auxin‐inactivating systems.