Southern Blight becomes active in midsummer. The first sign is yellowing leaves that fall over, followed by complete collapse of the plant. At the base of the plant a white webby mass forms with little tan seedlike sclerotia. It is caused by a fungus called Sclerotium rolfsii, which invades the petiole at ground level and liquifies it so it collapses. It spreads by the sclerotia, much like an annual weed. 
     To deal with it, first check to see if the sclerotia have formed. If they haven't, drench with a fungicide. If they have formed, it is very important to clean them up first. Remove the leaves carefully to a plastic bag and clean up all sclerotia, then drench with a fungicide Getting them all is vital to getting Southern Blight out of the garden, because it will spread and become an ongong problem. Digging is unnecessary and may spread the sclerotia.

     An article with more info and instructions is HERE.

In the Garden: A consumer product for spot treatment from Bayer called Disease Control for Roses, Flowers, and Shrubs with the active ingredient tebuconazole is effective. Bleach could be used in an emergency but is bad for the already-damaged plant.
In the Nursery: Professional products like Terraclor and Prostar are labeled and effective, both as spot treatment and preventative drenching.