Catalyzes the cyclization of geranylgeranyl diphosphate to casbene, a diterpene phytoalexin with antibacterial and antifungal activity. (2E,6E,10E)-geranylgeranyl diphosphate = casbene + diphosphate Binds 3 Mg(2+) ions per subunit. By oligogalacturonide fragments released by fungal infection. Detected after 5 hours of incubation with the pectic fragments and reaches a maximum after 10-12 hours. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. It is uncertain whether Met-1 or Met-8 is the initiator.