CHAPTER XX. 



GRASS MILDEW. 



Erysiphe graminis , D.C. 



THERE are few fungi more common or injurious in warm 

 dry weather than the fungus of grass mildew, grass blight, 

 or wheat rust, or white rust, as it is termed in America, 

 named Erysiphe graminis, D.C. It is possible that the 

 familiar straw blight of agriculturists, already described, 

 may be caused by the mycelium of the Erysiphe, and this 

 is an additional reason for directing special attention to 

 grass mildew or blight. The generic name Erysiphe was 

 the term given to mildew by the Greeks ; the specific 

 name graminis needs no explanation. The fine, creeping, 

 jointed mycelium of Erysiphe graminis, D.C., forms a 

 white superficial mildew on the living stems and leaves 

 of cereals and other grasses in the summer and autumn. 

 When the white mildew patches are examined in the 

 autumn with a very strong lens, they will be seen 

 sprinkled with minute black dots, as illustrated twice the 

 natural size on the wheat stem in Fig. 55. The spawn 

 of this mildew is generally supposed to be incapable of 

 penetrating the tissues of plants ; but suckers have been 

 described as belonging to the mycelia of some allied species 

 of mildew. With these minute suckers the Erysiphe 

 adheres to its host, if the suckers do not indeed pierce the 

 leaf cells and derive nourishment therefrom, after the 

 manner of Peronospora. In the summer the mycelium 

 gives rise to vast numbers of vertical moniliform, or neck- 

 lace-like groups or chains of conidia or spores, as illus- 

 trated at Fig. 56, enlarged 400 diameters. This peculiar 



