156 THE ROYAL SOCIETY OF CANADA 



although these same varieties were readily susceptible to a form of 

 rust collected at St. Paul, Minnesota. This susceptibility and 

 immunity of the same variety of wheat in different localities was 

 most readily explained by assuming the existence of more than one 

 biologic form of the rust fungus, each form capable of affecting only 

 certain wheats. 



Since 1916, by far the most extensive work on biologic specializa- 

 tion of P. graminis has been done by ^takman and his co-workers 

 (36, 37, 54, 55, 56, 57, 58, 59, 60) at the University of Minnesota, in 

 cooperation with the United States Department of Agriculture. 

 These investigators demonstrated conclusively the existence of about 

 a dozen biologic forms, which differ from one another chiefîy in their 

 infection reactions towards wheat varieties. Experiments were con- 

 ducted with about twenty-five strains and varieties of wheat, including 

 representatives of Triticum aestivum, T. durum, T. compactum, and 

 T. monococcum. All degrees of resistance and susceptibility to the 

 biologic forms then known were met with, from complete resistance 

 in Khapli (C.I. 4013) an emmer, to complete susceptibility in the club 

 wheats. The remainder of the varieties fell into an intermediate 

 group, susceptible in varying degrees to some forms of the rust, 

 and resistant or immune to others. 



To test the question of the possible temporary character or 

 mutability of these forms. Leach (36) and Stakman and Levine (60) 

 studied the behaviour of one of them, Piiccinia graminis tritici com- 

 pacti, and found it to be as constant parasitically as the forms originally 

 described by Eriksson (15). 



Recently Mains and Jackson (39) have shown that Puccinia 

 triticina Erik, consists of at least two biologic forms. 



In 1919, Hoerner (31) found evidence of the existence within oat 

 varieties of at least four specialized races in Puccinia coronata Cda., 

 which next to Puccinia graminis is probably the most extensively 

 investigated rust from the standpoint of heteroecism and biologic 

 specialization. 



Various workers have reported biologic specialization in the 

 sunflower rust, Puccinia helianthi Schw., but results have not been 

 conclusive until 1922, when Bailey (3) was able to demonstrate 

 definitely three distinct types of infection on Helianthus annuus, 

 supplying final proof of the existence of biologic forms of this fungus. 



Thus it is seen that a large number of rust species have been 

 investigated from the standpoint of specialization to particular hosts, 

 and the phenomenon has been found to be of wide occurrence. 



