390 BROOKLYN BOTANIC GARDEN MEMOIRS 



other cultures of the organism were used to inoculate the varieties a 

 different arrangement of susceptibility became evident. Varieties 

 quite immune to the one strain were severely attacked by another 

 strain. All the varieties proved quite susceptible to at least one of 

 the strains used. This indicates the existence of distinct races of this 

 parasite with fairly definite host limitations. 



Edgerton and Moreland (33) have made similar studies with cul- 

 tures of this same fungus. Their results also indicate differences in 

 the infecting capacity of strains of the fungus isolated from different 

 varieties of beans. Some beans, like the snap beans, appear to be 

 quite susceptible to a number of different strains. Other varieties, 

 while quite susceptible to certain strains, are resistant to strains from a 

 different source. 



Edgerton and Moreland have also studied cultures of Glomerella 

 gossypii (South.) Edg., the cotton anthracnose fungus. They do not 

 find evidence for the existence of specialized strains in this fungus, 

 for all the cultures isolated from different sources were able to infect a 

 large number of cotton varieties. 



General Discussion — Bridging Hosts 



The above review of the investigations with reference to host 

 specialization of parasitic fungi indicates that the phenomenon is of 

 general occurrence. The work done is particularly extensive in con- 

 nection with the rusts and the powdery mildews, but sufficient has 

 been accomplished in other groups to make clear the presence of 

 specialized races. 



It is highly probable that the same phenomenon is of wide occur- 

 rence among other groups of plant parasites. A large number of 

 so-called species of the Imperfect Fungi, as Cercospora, Phyllosticta, 

 and Septoria, may really be only specialized races of a relatively small 

 number of forms distinct on structural grounds. In several genera of 

 the Imperfect Fungi, as those mentioned, a very large number of 

 species have been recorded ; in fact many of them are, mainly at present 

 at least, identified by the host upon which they grow. Cultural ex- 

 periments may result in grouping many of these together, at the same 

 time making clear the physiological host rr^lations. 



We are not, however, to assume that host specialization is of 

 universal occurrence. There are several cases on record where the 

 fungus shows no evidence of the specialization of its hosts. A striking 

 case of this sort is that of Puccinia suhnitens Diet. Arthur (5) has 

 been able to infect ten hosts, belonging to the families Chenopodiaceae, 

 Cruciferae, and Capparidaceae, with teleutospores from Distichlis 

 spicata. Bethel (18), using teleutospores from the same grass, recently 



