112 



THE ASCOMYCETES 



sphaera, Podosphaera, Phvllactinia, Sphaerotheca, and Uncinula, 

 are generally recognized. They are separated on the bases of 

 type of cleistothecial appendages and number of asci within each 

 cleistocarp. 



Fig. 63. Stages in the initiation of cleistocarps in Erysiphaceae. A to F, 

 Sphaerotheca himnili. A. Antheridial and oogonial branches. B. Large 

 oogonium and small antheridium at apex of antheridial branch. C. Migra- 

 tion and association of nuclei of opposite sex; branches at base of oogonium 

 starting to form envelope. D. A later stage in which oogonium is enclosed 

 within wall. E. Oogonium has become a series of cells, the penultimate of 

 which is to become an ascus; contains a pair of nuclei that will fuse. F. 

 Young ascus containing fusion nucleus. Peripheral cells are nurse tissue. 

 G. Section of young cleistocarp of Erysiphe polygoni. (Adapted from 



Harper.) 



The appendages are flexuous, unbranched hvphae in Erysiphe, 

 Sphaerotheca, and Leveillula; they are unbranched but hooked 

 or coiled at the tip in Uncinula, dichotomously branched at least 

 at the tips in Microsphaera and Podosphaera, and needle-shaped 

 with bulbous bases in Phvllactinia. Each cleistothecium of 

 Podosphaera and Sphaerotheca contains only a single ascus. 



Mycelhmt and conidial stage. The name powdery mildew ap- 

 propriately describes the white, mealy appearance of affected 

 leaves, stems, flowers, and fruits, the whiteness being imparted 



