According to the field guide, this is the ‘bag-like volva’ part.
Anyway. You know how when you touch a dry mushroom, parts of it feels a bit soft and velvety? I wanted to convey that kind of texture to this portion, and decided to use Crewel wool and techniques to achieve this.
Using a Crewel needle, I used one strand of Appleton Bros worsted wool in three shades of mushroomy. I used Brick stitch for this, a satisfyingly regimented kind of long and short stitch, worked in horizontal bands, lovely for trying to convey the way that the Death Cap sort of bursts forth from it’s, well, it’s bag-like volva really.