Sweet sorghum (Sorghum bicolor (L.) Moench) has potential for bioenergy. It is adapted to a variety of U.S. locations and the extracted juice can be directly fermented into ethanol. However, little research on fungal stalk rots, diseases that pose serious constraints for yield and quality of juice and biomass, has been reported. A greenhouse bioassay was designed to assess charcoal rot (Macrophomina phaseolina) and Fusarium stalk rot (Fusarium thapsinum) in plants at maturity, the developmental stage at which these diseases are manifested. Multiple plantings of a susceptible grain line, RTx430, were used as a control for variation in flowering times among sweet sorghum lines. Lesion length measurements in inoculated peduncles were used to quantify disease severity. Sweet sorghum lines 'Rio' and 'M81E' exhibited resistance to F. thapsinum and M. phaseolina, respectively; and, in contrast, 'Colman' sorghum exhibited susceptibility to both pathogens. Lesion development over time in Colman was monitored. These results will enhance molecular and biochemical analyses of responses to pathogens, and breeding stalk-rot-resistant sweet sorghum lines.