Problems with Stevia

[slinky]

I love herb gardening, and Stevia is just such an interesting and tasty herb, so I have made a point of trying to keep one. Now, I'd like to think that I have a green thumb because I have never had any plant other than the stevia die. The bush that i had until just a few days ago has continually struggled with some strange fungus that almost selectively attacks it. The other plants near it have no problems with it. The leaves turn black rapidly and then fall off, and cutting off the afflicted plant parts does not stop it. The stem then turns black and powdery, sprouting some ashy mycelium- like growth (which is why i think that it is a fungus), and dies to the ground. It comes back sometimes, only to die back down. What is this fungus, and what can I do to stop it?

[slinky]

OK, this fungus is advancing/growing in strength. At first, it could only attack my stevia, but it wiped out my Ipomea Pes-caprae and my hydroponic garden, and attacks previously healthy plants. At first, it could produce no visible fuzz, just black, and was only able to attack the above ground parts of the stevia. Now, it can colonize any part of the plant with ferocity. My hydroponic garden was attacked by what I thought was a bacterial infection, because of blackening of roots, but then the whole plant withered and produced black spots and abberant white fuzzy growths all over it and the substrate. The roots, which almost always have water on or near them, covered in mold/fuzz while all the way submerged.
several of my Basils are completely de-foliated, and all cuttings from plants that I recently took exploded with the fungus, on their stems, on the leaves, and even completely grown throughout the soil in days. It killed a coleus, one of my hardiest plants. It's almost like it is evolving, or maturing.