Remove all infected material promptly and burn or send to landfill. This will reduce the production of airborne spores and prevent the formation of sclerotia (see the 'Biology' section below) to contaminate the soil.
Note that the first infections in spring and early summer start from spores released from germinating sclerotia. Good hygiene will reduce this risk, but since the spores are airborne and may blow in from elsewhere, it will not eliminate it. Later in summer there is an increased risk of infection as more airborne spores are released from infected plants.
There are no chemical controls available to gardeners.