Per Rod Tulloss, the white A. amerimuscaria do not constitute a monophyletic group. Therefore, this name is invalid.

I don’t think this name should be used unless someone is prematurely proposing a variety of a provisional name. We know that there are white variants of amerimuscaria, but we there is no evidence to indicate more than that some basidomes of the proposed taxon are occasionally white. Existing genetic evidence suggests that the white capped material that has been examined so far DON’T have a common ancestor.

Very best,

Rod