316 CLASS ASCOMYCETEAE 



species to be "homothallic." Yarwood (1935) on the other hand found the 

 sunflower mildew to be "heterothalHc." 



Salmon (1903, 1904a, b, c, 1905) in England, Reed (1916) in the United 

 States, and Hashioka (1938) in Japan, showed that some species are made 

 up of biological races which are confined to but a single host species or to 

 very closely related species. This is true of Erysiphe graminis, confined to 

 grasses (Poaceae) but in which the biological race on Poa will not infect 

 Bromus or Triticum and the strains on either of these genera will not 

 infect the other. Even for the genus Bromus there are some races of the 

 fungus that will attack certain species while other species are subject to 

 attack by other races. On the contrary Erysiphe ciclioracearum DC. is 

 very widespread in its host range and conidia from the fungus on one host 

 may infect many different hosts in families far apart systematically. 



In his very excellent monograph of this family Salmon (1900) recog- 

 nized six genera with a total of 49 species and 11 varieties in the whole 

 world. His species limitation is much more conservative and broad than 

 that of most later mycologists. Blumer (1933) recognized 80 species 

 in Central Europe alone besides mentioning 50 extralimital species. It is 

 therefore perhaps safe to say that there are upward of 150 species in the 

 world when all regions have been carefully explored mycologically. It 

 must be noted that the known species of the family are mainly confined 

 to the extratropical parts of the earth. As noted before, in the tropics the 

 not too frequently noted specimens are almost without exception in the 

 conidial stage, perithecia rarely occurring. 



In addition to the six genera recognized by Salmon in 1900 four more 

 have been added that like the others produce one-celled ascospores. The 

 author follows Theissen and Sydow (1917) in also including three more in 

 which the ascospores are two- or four-celled, the perithecia and mycelium 

 otherwise characteristic of the family. A key to these genera appears at 

 the close of this chapter. 



Several species of Erysiphaceae are harmful parasites of cultivated 

 plants. Sphaerotheca mors-uvae (Schw.) B. & C, of relatively minor im- 

 portance as a parasite of the berries of the American species of gooseberry 

 (Grossularia) , has become very destructive when introduced into Europe 

 where the European species of gooseberry are exceedingly susceptible to 

 injury by it. The same thing is true of Uncinula necator (Schw.) Burr., 

 which also is a minor enemy of the American species of grape (Vitis), but 

 when introduced into Europe proved very harmful to the susceptible 

 Vitis vinifera L. Podosphaera oxyacanthae (DC.) de Bary and P. leuco- 

 tricha (E. & E.) Salmon are sometimes harmful, especially to nursery 

 stock and young trees, to Prunus and Malus respectively, in America, 

 Europe, and other parts of the world. Sphaerotheca humuli (DC.) Burr, is 

 destructive to the hop {Humulus lupulus L.) wherever the host is grown 



