Southern Blight is endemic in the warm and wet areas of the world. The only sure answer is to stop growing the susceptible plant variety. Not very pleasant advice, I know. Since the question is usually what does Southern Blight NOT infect, the alternatives are few. Maize is the obvious one. But the rotation has to be a long one and it has to be accompanied by other measures such as the keeping of the infected ground as dry as possible, hoeing and burning on-site of all other growing material, implement and boot and hand-washing, using pig manure in large quantities, subject to the pH of the soil infected, applying lime or calcium nitrate to the maize. It is a nasty business.
Really, leaving the infected land alone and cleared of all growth for 4 years or so would be ideal but in SSA that is clearly not an option if there is no alternative site.
There are the usual fungicides such as captan and thiophanate methyl but they have to be applied long before the infection occurs and they are not a complete cure. Solarization can work a little prior to planting but the fungus always comes back. Really, apart from planting of maize for as many seasons as possible [ I know, it brings its own problems] the answer is to go for reduction of infection rather than elimination.