Catalyzes the NAD(H)-dependent interconversion of D-fructose 6-phosphate and D-mannitol 1-phosphate in the mannitol metabolic pathway. Has a strong preference for NADH over NADPH. Required for protection of conidiospores against exogenous stresses such as high temperatures and an oxidative environment. D-mannitol 1-phosphate + NAD(+) = beta-D-fructose 6-phosphate + H(+) + NADH Monomer. Strongly induced at the onset of sporulation. Belongs to the mannitol dehydrogenase family.