THE GRASS PERONOSPORA. 



429 



sporium ravenelii, (Curt)., that is often so abundant as to form a 

 dense, spongy mass. Wire grass is subject to similar attacks 

 from H. nodosum, (B. & C)., and other species of the same genus 

 occur on different grasses in a similar manner. 



FIG. 174. 



PERONOSPOREAE. 



12. The grass-peronospora, (Per- 



onospora graminicola, Sacc.) In the 

 leaves of Hungarian grass and pigeon 

 grass, ultimately filling them with a 

 snuff-brown, powdery mass. 



Hungarian grass (Setaria italica) is 

 sometimes attacked by a parasite clearly 

 related to the notorious potato blight 

 which forms a mycelium in the leaves of 

 the grass in the cells of which it lives. 

 Branches of this emerge sparsely through 

 the stomata and bear colorless conidia 

 which spread the disease. Later in the 

 season these spores are replaced by 

 winter spores (oospores) that originate 

 on branches of the mycelium within the 

 leaf by a process of fertilization. These 

 spores are contained in thick- walled, 

 brown envelopes, and presumably infect 

 new plants in the spring. So far, this 

 disease has not proved seriously destruc- 

 tive, though the leaves attacked are re- 

 duced to mere shreds when the winter 

 spores are ripe. The flower-clusters of 

 pigeon grass are greatly changed by the 

 fungus, according to Dr. Halsted. 

 Fig. 174.) 



