This is a serious disease, it's not worth trying to save the plant, it needs to go, and the sclerotia, and the soil it was in. Quoting from the above article, and others on the disease will say the same:
"As with many plant diseases, removal of infected plants is an important aspect of disease control. Sclerotium rolfsii usually causes infection at the lower stem (crown) section of the plant, and once infection takes place, removal of the whole plant is necessary. Prompt removal of infected plants will help prevent the addition of abundant fungal inoculum to the soil. If infected plants are allowed to remain in an area, these plants will serve as a continuous source of inoculum. Plant removal is a practical measure in landscapes, gardens, nurseries, and greenhouses, but not in field situations."
