INTRODUCTION 



The Erysiphaceae are popularly known in different countries as 

 ''White" or "Powdery Mildews/' -'Blight/' Mehlthau, Blanc, 

 Honungs-dagg", etc. Throughout the summer they are con- 

 spicuous in their '* Oidiuin^'' or conidial stage on many common 

 plants, e. g,^ roses, hops, vines, peas, maples, Plantago, Heracleinn^ 

 Polygonum^ Spiraea^ Coryhts^ Quercus, Crataegus^ etc., and give to 

 the infected parts of the host-plant a whitish mealy or dusty ap- 

 pearance due partly to the white web-like mycelium, and partly to 

 the presence of myriads of rapidly-formed white conidia. 



Later in the summer, and in autumn, the ascigerous form of 

 fruit is produced in the shape of small, more or less globular, 

 dark brown or black perithecia, usually provided with special out- 

 growths termed appendages. After producing these the mycelium 

 often dies away, and the perithecia are left as small, blackish, 

 globular bodies on the surface of the leaves and stems of the host- 



F 



plant. Sometimes, however, the mycelium Is thick and persistent, 

 and the perithecia are then found more or less immersed in it. 



The Erysiphaceae are characterized by the truly parasitic habit, 

 the white mycelium, the production of large, colorless (or white) 

 non-septate conidia on simple, erect conidiophores (forming the 

 Oidium stage), and the indehiscent perithecia, or cleistocarps 

 (mostly provided with appendages of a very definite form), con- 

 taining non-septate ascospores. The family thus hmited contains 

 the genera Podosphacra^ SpJiacrotJieca^ Uncimila, Microsphacra, 

 Erysiphe, and Phyllactinia. 



^^emoirs Torrey Botanical Club, Volume IX. 



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