We investigated ascospore discharge in the perithecial fungus, Gibberella zeae. In a wind tunnel study that simulated constant rain and varying day and night lengths, the rate of ascospore release was approximately 8-30% greater under light than in complete darkness. Under constant light, ascospore discharge occurred at maximal rates at relative humidity levels greater than 92%. When perithecia were placed under conditions of high external osmolarity, ascospore discharge was significantly reduced. Ascospores were discharged from asci along with droplets of fluid, the epiplasm, from within the ascus. Analysis of discharged epiplasmic fluid by GC-MASS Spectrometry revealed that mannitol was the major simple sugar component of the fluid. Activity of mannitol dehydrogenase, which catalyzes the conversion of fructose to mannitol, was higher in protein extracts from mature perithecia than in extracts from vegetative tissue. Several inhibitors of K(+) and Ca(++) ion channels inhibited ascospore discharge, which suggested that ascospore discharge resulted from the buildup of turgor pressure generated by ion fluxes and mannitol accumulation.