Involved in momilactone phytoalexins biosynthesis. Catalyzes the last step of momilactone A biosynthesis. 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide + NAD(+) = H(+) + momilactone A + NADH 3beta-hydroxy-9beta-pimara-7,15-dien-19,6beta-olide + NADP(+) = H(+) + momilactone A + NADPH By chitin oligosaccharide elicitor and UV irradiation. The phytoalexins momilactones A and B are diterpenoid secondary metabolites involved in the defense mechanism of the plant and produced in response to attack (by a pathogen, elicitor or UV irradiation). Momilactone B can also act as an allochemical (an antimicrobial and allelopathic agent), being constitutively produced in the root of the plant and secreted to the rhizosphere where it suppresses the growth of neighboring plants and soil microorganisms. Belongs to the short-chain dehydrogenases/reductases (SDR) family.