Aims Ripe, fleshy fruits generally function as rewards to attract mutualistic seed dispersers, but many fruits also contain high concentrations of toxic secondary metabolites. These compounds may serve a variety of adaptive roles in seed dispersal or as a defense against non-dispersing seed predators or pathogens. We tested the effects of iridoid glycosides from fruits of a hybrid bush honeysuckle, Lonicera × bella, on the growth of two pathogenic fungal strains associated with fruit rot, Alternaria tenuissima and Aspergillus tubingensis.
Methods Fungi were isolated from field-collected L. × bella fruits and identified using molecular techniques. Their growth rates were assessed in vitro in the presence of varying concentrations of pure loganin, one of the most abundant iridoid glycosides in fruits, as well as fruit extracts containing a mix of at least seven different iridoid glycosides.
Important findings Loganin had strong dose-dependent negative effects on the growth of both fungi. Extracts from fruits had no effect on Aspergillus but a strong antifungal effect on Alternaria that increased with fruit ripening. Total iridoid glycoside concentrations in extracts were not good predictors of variation in fungal growth, but several individual compounds had significant negative effects. Although iridoid glycosides have primarily been studied as antiherbivore defenses in leaves, these results indicate that they can also function to reduce the growth of fungi associated with fruit rot.