I'd say it's a 'fungal" type disease.....maybe 'crown rot' ?
http://extension.psu.edu/pests... there's another site wit better / more pictures, and a more 'encouraging' treatment....but I can't find it right now
.
I also found two with rot problems, today......the 'stinky, soft rot".....I scraped, and squirted with Listerine, then drenched one plant with a bleach /anti bacterial dish soap mixture, and gave the other two aspirin
.Neither was 'terrible', but, ANY problems are bad enough !!
I'll check again for that other disease page, but, if it was me, I'd be sure to include some sort of fungicidal treatment, rather than just a biocide.
