Powdery mildew disease of ornamental, crop and other plants is caused by a large number of fungi species in the family Erysiphaceae. This family is divided into six main genera that are identifiable from one another by their differing cleisothecial appendages and varying numbers of asci per cleistothecium.

 
Key to the Genera of the Powdery Mildew Fungi

1. Cleistothecial wall thin and hyaline…....................................…..….Brasiliomyces
1. Cleistothecial wall thick and dark brown….........................……….2

2. Cleistothecia containing a single ascus…….........................……….3
2. Cleistothecia containing several to many asci.......................……….4

3. Cleistothecial appendages mycelioid (non-branched)……...……….Sphaerotheca
3. Cleistothecial appendages dichotomously branched….................….Podosphaera

4. Setose mycelial cells formed around the cleistothecia...............…….Blumeria
4. Setose mycelial cells absent………………………............……….5

5. Cleistothecial appendages mycelioid or lanceolate…..........………..6
5. Cleistothecial appendage tips dichotomously branched or curved......7

6. Cleistothecial appendages mycelioid………….................…………Erysiphe
6. Cleistothecial appendages rigid, tapered with bulbous base….….….Phyllactinia

7. Cleistothecial appendage tips dichotomously branched….......………Microsphaera
7. Cleistothecial appendage tips curved or coiled……..........…………Uncinula