198 GRASSES OF IOWA. 



is certainly very expressive of its appearance. An examina- 

 tion with a microscope will show that this white substance is 

 composed of spores and a mycelium. The mycelium is cob- 

 webby and spreads over the surface, but does not penetrate 

 the leaf. In numerous places erect branches are produced, 

 these bear numerous spores This stage was formerly called 

 Oidium moniUoides. Called Oidium because the spores resemble 

 an Qgg. In all cases of Oidium this is not true; the species wds 

 called monilioides because necklace like, referring to the man- 

 ner in which the spores are borne. Worthington G. Smith 

 states the spores are so numerous that it would take about a 

 million spores to cover a square inch. 



These conidia or summer spore 5 germinate, under favorable 

 conditions, in from ten to sixteen hours. The temp?rature 

 most favorable for germination is from 17-26 C. In a powdery 

 mildew occurring on the squirrel-tail grass, and supposed to 

 be the same fungus, these spores are also capable of immediate 

 germination. On blue grass the fungus frtquently does not 

 produce perithecla but ends its existence with the formation of 

 conidia. 



Under favorable conditions, especially moisture and damp 

 weather, the fungus spreads rapidly. The leaf of grass 

 affected by this fungus soon dries up. When the leaves have 

 become dry and the attested plants are disturbed a little, clouds 

 of dust arise, especially in shady places. The perfect stage of 

 the fungus is not of common occurrc^nce, though if careful 

 search is made in the fall, small black specks may be seen ; these 

 are the peri hecia and contain the asci a -id ascospores. It is the 

 resting stage or winter condition of the fungus. The writer 

 found the perfect fungus abundant on Poa loolfii in Colorado, 

 and Carver found it abundant on blue grass near Ames one 

 season. The Oidium stage does not reta'n its power of ger- 

 mination very long, but the ascospores contained in th^ peri- 

 thecium germinate the folio ving spring, and when the lube 

 comes in contact with the proper host the mycelium spreads 

 over the surface of the leaf and causes the mealy appearance. 

 Plants ajf'ected. — It affects especially blue grass with us and 

 is much worse some seasons than others. English investi- 

 gators report that this fungus is most abundant when slight 

 frosts occur, also with heavy rains and wet soils. Texas blue 

 grass {Poa arachnifera), fowl meadow grass {Poa serotina), 

 Eatonia obtusata, and red top {Agrostis alba vulgaris) and many 



