Southern blight is a warm climate disease for the most part, it's not common where there are cold winters. The hard thing is to kill the sclerotia (the mustard seeds), this fungus does not produce asexual spores. The pH change does sound plausible although it might be better for the soil to use lime if calcium is low. I did find this article that seems relevant, haven't had time to read every word though.
http://www.apsnet.org/publicat...
