IOWA ERYSIPHACEAE. 
BY J. P. ANDERSON. 
INTRODUCTION. 
The Erysiphacese are popularly known as blights, white or 
powdery mildews. They are parasitic on quite a large variety of 
plants and during the summer are often quite conspicuous factors 
of the vegetation, covering the leaves or other parts with a white 
mycelium which sometimes gives the host an almost hoary ap- 
pearance. 
The plant body proper consists of numerous branching, septate, 
usually white, much interwoven threads called the mycelium. These 
threads are superficial and adhere to and derive nutriment from 
the host by means of haustoria which arise from the mycelial 
threads and penetrate the epidermis of the host. An exception to 
this is found in Phyllactinia in which special branches of the my- 
celium enter the stomata of the host and send haustoria into the 
surrounding cells. 
During the summer and early fall the asexual reproductive bodies 
or conidia are formed. They are cylindrical, oval or barrel-haped, 
colorless cells filled with protoplasm. They are formed by constric- 
tion at the ends of short, erect, simple, club-shaped, septate, color- 
less branches of the mycelium known as fertile hyphae or conidio- 
phores. The conidia are often found in chains, several from the end 
of the same hypha having fallen away together. The conidia are 
produced in immense numbers throughout the growing season, are 
light and easily carried by the wind, and serve for the rapid increase 
and wide distribution of the parasite as they germinate quickly 
under favorable conditions. In germinating the conidium sends 
out a slender tube which upon the proper host and under the proper 
conditions, soon develops into a new mycelium. 
Later in summer or in autumn the true reproductive bodies or 
perithecia are formed. These perithecia contain the ascopores or 
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