THE FUNGI WHICH CAUSE PLANT DISEASE 171 



Fig. 122. — ^I, E. graminis, showing branching 

 haustoria. 33, Phyllactinia, intercellular 

 hyphs. After Smith. 



Erysiphaceae (p. 170) "■ "• "• " 



This family on account of its abundance everywhere, its sim- 

 plicity of structure, and its possession of typical ascigerous and 

 conidial stages forms a favorite type for introductory study of the 

 Ascomycetes. Its members 

 are easy of recognition, form- 

 ing a coating of white conidia, 

 conidiophores and mycelium 

 upon the surface of its hosts 

 and giving them an appear- 

 ance much as though they had 

 been lightly dusted with flour. 

 Later in the season the white 

 patches are more or less 

 liberally sprinkled with the 

 black perithecia leading to 

 the conmion name powdery 

 mildew. An important list 

 of the economic forms and their hosts has been published by 

 Halsted." 



The mycelium except in Phyllactinia is entirely superficial. It 

 is usually quite hyaline and is branched, septate and its cells 

 uninucleate. It fastens to the host and penetrates its epidermal 

 cells by uninucleate haustoria which by their various lobings aid 

 in specific characterization. Figs. 122, 123. 



Haustoria may be grouped in three general classes; (1) those 

 arising directly from the lower surface of the mycelium; (2) those 

 arising at the side of the mycelium as small semicircular processes; 

 (3) arising from more or less deeply-lobed lateral swellings of the 

 mycelium. The relation of the haustoria to the host cells has 

 been extensively studied by Smith.™ 



The conidia arise in basipetal succession on simple scattered 

 conidiophores (Fig. 129); are hyaline, oval or barrel-shaped, 

 smooth, 1-celled. Neger has shown that they vary greatly in size 

 with nutrition conditions.*" 



Conidia germinate readily at once in dry air, better in himiid 

 air, producing from one to three germ tubes. Haustoria are 



