Highly reducing polyketide synthase; part of the gene cluster that mediates the biosynthesis of pyriculol and pyriculariol, two heptaketides that induce lesion formation upon application on rice leaves but are dispensable for pathogenicity (PubMed:27902426). The highly reducing polyketide synthase synthesizes the heptaketide backbone of pyriculol and pyriculariol (PubMed:27902426). Pyriculol and pyriculariol contain several hydroxyl moieties and double bonds, so it can be assumed that several reduction steps occur during biosynthesis. These reactions could be executed by PKS19 itself or partly by the tailoring enzymes OXR1, PXR2, RED1, RED2 or RED3, identified within the cluster (Probable). The FAD-linked oxidoreductase OXR1 is the only tailoring enzyme for which the function has been determined yet, and is involved in the oxidation of dihydropyriculol and dihydropyriculariol into pyriculol and pyriculariol, respectively (PubMed:27902426). Polyketide biosynthesis. Expression is increased by fivefold in rice-extract medium (REM) and is correlated with the production of pyriculol (PubMed:27902426). Expression is also increased during invasive growth during rice infection (PubMed:27902426). Expression is negatively regulated by the 2 cluster-specific transcription factors TRF1 and TRF2 (PubMed:27902426). 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; 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. Abolishes the production of pyriculol, pyriculariol, as well as of their dihydro derivatives dihydropyriculol and dihydropyriculariol.