Catalyzes the NAD(+)-dependent oxidation of geranial to geranic acid. Is involved in the anaerobic degradation of the monoterpene beta-myrcene. Seems to be specific for the trans-isomer geranial, since it does not act on the cis-isomer neral. (2E)-geranial + H2O + NAD(+) = geranate + 2 H(+) + NADH Terpene metabolism; monoterpene degradation. By the monoterpene phellandrene, but not by acetate. Belongs to the aldehyde dehydrogenase family.