272 MANUAL OF POISONOUS PLANTS 



Poisonous properties. The Veterinarians of Europe ascribe to these mil- 

 dews a form of stomatitis. 



Erysiphe graminis D C. 



Amphigenous, often epiphyllous, mycelium dense, felt-like, persistent, white 

 or gray, sometimes tinted brown ; perithecium immersed in the mycelium, few 

 and scattered, large, about 225 M, in diameter ; asci 16-25 M oblong or oval, 

 stalked, ascospores 8 or rarely 4; appendages rather short. 



Distribution and Hosts. Found on many different grasses like blue grass 

 (Poa pratensis), fowl meadow grass (Poa serotina), occasionally also on wheat 

 or orchard grass. The following rather popular account treats of this disease 

 as it is common in the west. 



Every one who has had occasion to walk through a blue grass meadow 

 after a rain, especially in damp and shaded places close to the ground, must 

 have noticed a white mealy covering on the blades of many of the leaves. The 

 Germans have called this mehlthau (literally translated meal dew), which is 

 certainly very expressive of its appearance. An examination with a microscope 

 will show that this white substance is composed of spores and a mycelium. 

 The mycelium is cobwebby and spreads over the surface, but does not pene- 

 trate the leaf. In numerous places erect branches are produced, these bear 

 numerous spores. This stage was formerly called Oidium monilioides, being 

 named Oidium because the spores resemble an egg, although the resemblance 

 is not marked in all cases of Oidium; the species was called monilioides because 

 it was necklace like, referring to the manner in which the spores are borne. 

 Worthington G. Smith states that the spores are so small that it would take 

 about a million to cover a square inch. 



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 frequently does not produce perithecia but ends its 

 existence with the formation of conidia. It produces perithecia abundantly on 

 wheat in Iowa. 



These conidia or summer spores germinate, under favorable conditions, 

 in from ten to sixteen hours. The temperature 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 im- 

 mediate germination. 



Under favorable conditions, especially moisture and damp weather, the 

 fungus spreads rapidly. The leaf of grass affected by this fungus soon dries, 

 and when the affected plants are disturbed, small clouds of dust arise, especi- 

 ally in shady places. The perfect stage of the fungus is not of common oc- 

 currence, though if careful search is made in the fall, small black specks may 

 be seen ; these are the perithecia and contain the asci and ascospores. It is 

 the resting stage or winter condition of the fungus. The writer found the 

 perfect fungus abundant on Poa Wolfii in Colorado, and Carver found it 

 abundant on blue grass near Ames one season. The spores of the Oidium 

 stage do not retain their power of germination very long, but the ascospores 

 contained in the perithecium germinate the following spring, and when the 

 tube comes in contact with the proper host the mycelium spreads over the 

 surface of the leaf and causes the mealy appearance. 



