Catalyzes the conversion of syn-copalyl diphosphate to the phytoalexin precursor stemarene. 9alpha-copalyl diphosphate = diphosphate + stemar-13-ene Binds 3 Mg(2+) ions per subunit. By chitin oligosaccharide elicitor and UV irradiation. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Stemar-13-ene is a putative precursor of the phytoalexin oryzalexin S. Phytoalexins 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). May be due to a competing acceptor splice site. Belongs to the terpene synthase family.