White mold is a disease caused by several species of the fungus sclerotinia. The severity of the disease can vary and may range from mild and barely noticeable to deadly. Some plants seem to be more susceptible to white mold--petunias, lima beans and sunflowers among them. White mold manifests itself in different ways, the most obvious of which is a white, powdery substance on plants and on the soil. There is no cure for the disease; removal of infected plants must be done to prevent the spread of the disease to neighboring plants. There are, however, steps you can take to prevent white mold in the future.
Prune the plant 4 to 5 inches below the infected portion. If any plants are too infected for this procedure, remove them from the garden and burn them.
Remove the top 2 to 3 inches of soil and replace it with fresh soil.
Remove any weeds in the area as they, too, can be infected with white mold.
Prevent further infections by clearing away all dead leaves and other debris on the soil, watering less frequently and applying fungicides with preventive properties. Your county extension agent will be able to give you information on which fungicide is suitable for your area.