Inhibition of spore germination and appressorium formation of rust species by plant and fungal metabolites

Eight fungal and plant metabolites belonging to different classes of naturally occurring compounds, a 24-oxa[14]-cytochalasan as cytochalasin B (1), a trisubstituted isocoumarin as 6-hydroxymellein (2), a tetracyclic pimarane diterpene as sphaeropsidin A (3), a chalcone as cavoxin (4), a pentasubstituted benzofuranone as cyclopaldic acid (5), a bicyclic-sesquiterpene as inuloxin A (6), a epipolythiopiperazine as gliotoxin (7) and a cyclohexene epoxide as epiepoformin (8), were tested for their effectiveness in reducing early stages of development of several major rust fungi from the genera Puccinia and Uromyces. Spore germination and appressoria formation were assessed on pre-treated detached leaves, under controlled conditions. Among the various metabolites evaluated, compounds 5 and 8 were the most effective in inhibiting fungal germination and penetration of all rust species studied at values comparable with those obtained by fungicide application, while compound 4 was phytotoxic to plant leaves at any concentration tested