6 Raymond J. Pool 



quisqualis Caseti., which is parasitic upon the mildew. Some- 

 times the perithecia are filled with the fruits of the parasite to the 

 exclusion of the asci. This parasite apparently attacks all of 

 the species in the group, and when abundant probably serves in 

 a measure to check the development of the particular mildew 

 infected. ' It is possible that the vitality of the mildew is so 

 lessened by such an attack that its spread is considerably checked. 

 The Erysiphaceae are of considerable economic importance as 

 plant diseases. In some localities they become quite destructive. 

 The pathological phase of the group is left for the plant patho- 

 logist. Suggestions along this line with directions for their pre- 

 vention or control may be found in many Agricultural Experi- 

 ment Station publications, and in more extensive treatises on 

 plant pathology in general. 



ERYSIPHACEAE Lev. 



Fungi parasitic on seed plants ; epiphytic or partially endo- 

 phytic ; mycelium white, or in some cases becoming brownish ; 

 haustoria formed in the epidermal cells from external hyphae, or 

 within deeper lying cells from special hyphal branches sent 

 through the stomata into the intercellular air spaces ; hyphae thin- 

 walled, septate, much branched and interwoven ; conidia large, 

 continuous, hyaline, cylindrical, oblong, or barrel-shaped, pro- 

 duced singly or in chains on erect, simple, septate, colorless spor- 

 ophores ; perithecia on the mycelium, at first colorless, then yel- 

 low to brown, finally black, membranous or carbonous, without 

 an ostiole, globose or globose-depressed, sometimes becoming 

 lenticular or concave ; asci one to many, arising from the base of 

 the perithecium, colorless, cylindrical, oblong, ovate, or globose, 

 frequently pedicellate, 2-8-spored ; spores continuous, colorless, 

 oblong or oval, straight or slightly curved. 



KEY TO THE GENERA 



I. Perithecia containing a single ascus ; asci mostly 8-spored. 



1. Appendages dichotomously branched at the tip. . . . Podosphaera 



2. Appendages simple or obscurely branched S phaerotheca 



II. Perithecia containing several asci; asci 2-8-spored. 



3. Appendages acicular, bulbous at the base Phyllactinia 



4. Appendages uncinate or coiled at the tip Uncinula 



5. Appendages dichotomously branched at the tip Microsphaera 



6. Appendages simple or obscurely branched Erysiphe 



64 



