Several ionones and β-ionylideneacetic acids inhibited absicisic acid (ABA) biosynthesis in Cercospora rosicola at 100 μM. At lower concentrations, α-ionone, 1', 2'-dihydroxy-1', 2'-dihydro-β-ionone and 4'-keto-α-ionone enhanced ABA biosynthesis. Conversions of ionones by C. rosicola were identified by GC-MS as: α-ionone to 4'-keto-α-ionone, 4'-keto-α-ionol and dehydrovomifoliol; and 1'-hydroxy-α-ionone to dehydrovomifoliol. The oxidations of α-ionone and its analogs parallel those of the α-ionylideneacetic acids. The β-ionylideneacetic acids were generally oxidized to more polar forms. Metabolites identified by GC-MS were 3'-hydroxy-, 3'-keto- and 1', 2'-epoxy-1', 2'-dihydro-β-ionylideneacetic acids. The fungus rapidly metabolized most exogenous materials to more polar forms.

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