It is caused by " Sphaerotheca humuli ", a fungus in the order Erysiphales.
2.
*" Erysiphales " are notable for intricate appendages which follow fractal geometry within Fibonacci numbers and can be useful for species identification.
3.
In horticulture, mildew is either species of fungus in the order Erysiphales, or fungus-like organisms in the family " Peronosporaceae ".