A basal stem rot fungus can attack sedums near ground level. The fungus survives in moist soil, and can splash up onto the stems. Providing better air movement around the plants along with full sun to dry things out is one means of getting the problem under control. If plants are crowded, they may need dividing. Mulch the soil to prevent soil from splashing on the stems. Fungicide sprays can also help prevent the disease from developing.