216 FUNGOUS DISEASES OF PLANTS 



commonly known as mildews, powdery mildews, and blights 

 (Germany, Mehltau ; France, blanc, etc.). Some writers would 

 make more than a hundred species of the various forms, the 

 species being determined very largely by the hosts upon which 

 they occur. The Erysiphaceae are all strictly parasitic, producing 

 a considerable, septate, superficial mycelium with a single form 

 of conidial spore and a closed perithecium containing the asci. 

 This family is such a homogeneous, coherent group that it may 

 be treated as a whole, and subsequently a few notes on particularly 

 important species may be made. 



Geographical. The various members of this family are, gener- 

 ally speaking, most abundant in the north temperate regions of 

 the earth, but as a family they are -not limited in their distribu- 

 tion. Moreover, one species is known to occur as far north as 

 Greenland, while another is found in Terra del Fuego. The 

 number of species common to America, on the one hand, and to 

 Europe, Asia, and Africa, on the other, is approximately the same ; 

 but there are supposedly more endemic forms in America than in 

 all other countries. Salmon gives fourteen endemic species with 

 five varieties for America, while only thirteen species and four 

 varieties are known to be endemic in Europe, Asia, and Africa 

 combined. 



Climatic relations. The distribution of these fungi is ap- 

 parently not closely restricted by slight climatic differences. A 

 certain amount of moisture is unquestionably essential to the 

 vigorous production of the superficial mycelium characteristic 

 of this group, and there are fewer species in dry, exposed 

 regions, as, for instance, in the Great Plains regions of the 

 United States, than in the more moist Appalachian region. 

 Nevertheless, there are a number of species that may be found 

 from the extreme north to the extreme south, as well as from 

 east to west in both the eastern and western continents. Climatic 

 conditions, especially, may determine whether or not a particu- 

 lar species may become a devastating disease-producing organism 

 or may be classed merely as a fungus of occasional economic im- 

 portance. Erysiphe graminis, for instance, is seldom a fungus of 

 any consequence in most sections of the United States, while in 

 England it may at times cause serious injury to cultivated grasses. 



