Gabe,
It's Mustard Seed Fungus, aka 'Southern Blight' and sometimes referred to as 'crown rot'.
I've followed some articles on this from a few years back, and it occurs in hot, humid
conditions. I use a fungicide in powdered form called "Terrachlor" which controls it here.
I got my supply several years ago, and I am not even sure it's still available. (Just googled
it; and it is available -apparently- check this link:
http://www.lawnandlandscape.co...
Hope this helps.
