I don't know much about mustard seed fungus, except that you'll find kind of a cottony substance in the folds of the leaves, followed by spherical fruiting body that resembles mustard seed.
http://www.ipm.ucdavis.edu/PMG...
http://www.irises.org/About_Ir...
There's a good picture of the fruiting structure and white moldy growth on a peanut stem.
http://ipm.ncsu.edu/peanuts/di...
I was told to scrape out the soft part until you get to healthy tissue, then soak in a ten percent bleach solution. Swish it around so that you get in the leaves, and I pulled the leaves apart a bit to make sure the bleach solution got in there. Then dry for a day or two before replanting. It was recommended that I replace the soil where it was growing as well. I'd say that's pretty much the advice for other fungal rots as well.