The Erysiphaceae of Nebraska 3 



acter which does not exist elsewhere in the family. In the later 

 stages of the conidia production mycelial hyphae emerge in great 

 numbers from the stomata, and form an external mycelium on 

 the surface of the leaf very similar to the mycelium of the other 

 species of the group. Upon this mycelium the perithecia are de- 

 veloped. On account of these peculiarities, Salmon considers 

 that Erysiphe taurica differs generically from the other mem- 

 bers of the group, and he has given the name Oidiopsis to this 

 particular form. He also proposed a new sub-family, Oidiop- 

 sidae, for the reception of this genus. 



There are two kinds of reproductive bodies formed in the Ery- 

 siphaceae, the asexually-produced conidia or conidiospores, and 

 the ascopores formed in perithecia. The conidia are formed 

 under the favorable conditions prevailing throughout the sum- 

 mer and early autumn. From the mycelium arise erect, simple 

 hyaline or white branches which are very delicate, thin-walled, 

 one- to many-septate, and about 10/x thick and from 120-380^, 

 high. The conidia are formed at the tips of these branches 

 (Plate I, fig. 2) either singly or in long chains in basipetal suc- 

 cession. The conidia are continuous, colorless or white, oblong, 

 cylindrical or barrel-shaped, smooth, thin-walled and 18-50 X 

 9-25/x in size. 



The conidia are formed in immense numbers throughout the 

 summer. They are easily carried by the wind, and are capable 

 of immediate germination (Plate I, fig. 3) on reaching a suitable 

 host. The conidia are therefore excellent devices for the rapid 

 spread of the fungus during the growing season. 



These conidial forms were formerly classified as separate spe- 

 cies of the genus Oidium, a supposed autonomous genus of the 

 hyphomycetes. By the researches of Berkeley and Tulasne from 

 1840-1855 it was proved that an organic connection exists be- 

 tween the mycelium which produces the conidia and that which 

 later in the season bears the ascigerous fruits. 



The second kind of reproductive body of the Erysiphaceae is 

 formed and matured much later in the autumn, at a time when 

 the general tone of the host plant is on the decline. The conidia 



61 



