Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of abscisic acid (ABA), a phytohormone that acts antagonistically toward salicylic acid (SA), jasmonic acid (JA) and ethylene (ETH) signaling, to impede plant defense responses (PubMed:31034868). The first step of the pathway catalyzes the reaction from farnesyl diphosphate to alpha-ionylideneethane performed by the alpha-ionylideneethane synthase abl3 via a three-step reaction mechanism involving 2 neutral intermediates, beta-farnesene and allofarnesene (By similarity). The cytochrome P450 monooxygenase abl1 might then be involved in the conversion of alpha-ionylideneethane to alpha-ionylideneacetic acid (By similarity). Alpha-ionylideneacetic acid is further converted to abscisic acid in 2 steps involving the cytochrome P450 monooxygenase abl2 and the short-chain dehydrogenase/reductase abl4, via the intermediates 1'-deoxy-ABA or 1',4'-trans-diol-ABA, depending on the order of action of these 2 enzymes (By similarity). Abl2 is responsible for the hydroxylation of carbon atom C-1' and abl4 might be involved in the oxidation of the C-4' carbon atom (By similarity). Pks5 is clearly not involved in the production of ABA. Nonetheless, the possibility cannot be excluded that pks5 may modify ABA into another compound. It also cannot be excluded the possibility that pks5 also has a function completely independent of ABA synthesis (PubMed:31034868). Pks5 is not required for pathogenicity on B.napus cotyledon (PubMed:31034868). Expression is induced during the early biotrophic stage of development (PubMed:31034868). Expression is positively regulated by the ABA cluster-specific transcription regulator abl7 (PubMed:31034868). Multidomain protein; including a ketosynthase (KS) that catalyzes repeated decarboxylative condensation to elongate the polyketide backbone; a malonyl-CoA:ACP transacylase (MAT) that selects and transfers the extender unit malonyl-CoA; a dehydratase (DH) domain that reduces hydroxyl groups to enoyl groups; a methyltransferase (CMeT) domain responsible for the incorporation of methyl groups; an enoylreductase (ER) domain that reduces enoyl groups to alkyl group; a ketoreductase (KR) domain that catalyzes beta-ketoreduction steps; and an acyl-carrier protein (ACP) that serves as the tether of the growing and completed polyketide via its phosphopantetheinyl arm. Does not affect the production of abscisic acid (ABA) but leads to the accumulation of unidentified compounds (PubMed:31034868). Does not affect sporulation, pycnidiospore germination nor fungal growth rates in vitro, and does not alter the pathogenicity of L.maculans (PubMed:31034868).