<jats:title>Abstract</jats:title><jats:p>The specialized, fungal pathogen <jats:italic>Escovopsis weberi</jats:italic> threatens the mutualistic symbiosis between leaf‐cutting ants and their garden fungus (<jats:italic>Leucoagaricus gongylophorus</jats:italic>). Because <jats:italic>E. weberi</jats:italic> can overwhelm <jats:italic>L. gongylophorus</jats:italic> without direct contact, it was suspected to secrete toxins. Using NMR and mass spectrometry, we identified several secondary metabolites produced by <jats:italic>E. weberi. E. weberi</jats:italic> produces five shearinine‐type indole triterpenoids including two novel derivatives, shearinine L and shearinine M, as well as the polyketides, emodin and cycloarthropsone. Cycloarthropsone and emodin strongly inhibited the growth of the garden fungus <jats:italic>L. gongylophorous</jats:italic> at 0.8 and 0.7 μmol, respectively. Emodin was also active against <jats:italic>Streptomyces</jats:italic> microbial symbionts (0.3 μmol) of leaf‐cutting ants. Shearinine L instead did not affect the growth of <jats:italic>L. gongylophorus</jats:italic> in agar diffusion assays. However, in dual choice behavioral assays <jats:italic>Acromyrmex octospinosus</jats:italic> ants clearly avoided substrate treated with shearinine L for the garden fungus after a 2 d learning period, indicating that the ants quickly learn to avoid shearinine L.