ERYSIPHACEAE 173 



by the profusion of external mycelium and conidia. The arach- 

 noid mycelium, composed of uninucleate cells, with few excep- 

 tions is external to the host tissues. The host cells are penetrated 

 by haustoria, one or more entering each epidermal cell. In Uji- 

 cimila salicis the penetrating tubes may also pass through the epi- 

 dermal cells and form haustorial expansions within the cells of the 

 mesophyll [Smith (1900)]. In most species the haustoria are 

 bulbous and uninucleate. In Erysiphe graj/imis, however, and 

 in certain other species of Erysiphe, notably E. galiopsidis and E. 

 dehor acearimty they are ellipsoid, with digitate processes at the 

 ends [Smith (1900)]. 



Some species of powdery mildews perennate as mycelium. 

 Apple powdery mildews, caused by Podosphaera leucotricha and 

 F. oxyacajithae, have been reported to hibernate between the bud 

 scales [Ballard (1914)]. Similarly a series of observations has 

 convinced the writers that the powdery-mildew fungus of crepe 

 myrtle, Lagerstroemia indica, perhaps JJjicimda lagerstroemiae, 

 overwinters as mycelium within the buds. 



Oidiopsis (Leveilhda) taiirica is of particular interest because 

 its mycelium is wholly endophytic [Salmon (1906)]. This 

 fungus is endemic to Asia Minor and northern Africa, occurring 

 on many species of plants. This endophytic habit is believed to 

 be an xerophytic adaptation. Its conidiophores emerge through 

 the stomata. In like manner a portion of the mycelium of Fhyl- 

 lactinia corylea occurs between the mesophyll of its numerous 

 hosts. 



The conidial stage of most powdery mildews belongs to the 

 form Genus Oidium. Special hyphal branches arise from the 

 leaf surface, and near the tip of each a septum is formed. This 

 apical cell becomes the first conidium in a chain of basipetally 

 abstricted conidia. Each conidium is at first cylindrical, then 

 becomes barrel-shaped, and finally, when ready for dissemina- 

 tion, is elliptical. If moisture is provided, conidia germinate by 

 formation of one or more germ tubes, and within 3 or 4 days new 

 mycelium has grown to the extent of being capable of forming 

 a crop of conidia. 



Cleistothecial stage. The origin of cleistocarps of powdery 

 mildews was properly traced and their structure correctly de- 

 scribed by de Bary as long ago as 1863. He employed Sphae- 

 rotheca castagnei growing on dandelion and noted that at the 



