Involved in the biosynthesis of tri- and tetraketide alpha-pyrones. Pks18 catalyzes the extension of medium- and long-chain aliphatic acyl-CoA substrates by using malonyl-CoA as an extender molecule to synthesize polyketide products. Lipid metabolism; fatty acid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.