S34 THE BASIDIOMYCETES 



lief in the mycoplasm hypothesis, followed. When he grew 

 cereals in the greenhouse, the plants became infected. It seems 

 his^hlv probable, however, that Eriksson made use of diseased 

 seed grain and that insufficient care was exercised to prevent in- 

 fection in his greenhouse experiments. 



Specialization 



Specialization of parasitism among rusts constitutes one of the 

 most interesting and important problems in uredinology. A basic 

 concept in this field requires that a species be regarded as a col- 

 lection of morphologically similar organisms or strains that differ 

 in ability to produce infection in different hosts. Although this 

 matter is more fully discussed in Chapter 11, Volume II, it should 

 be indicated at this point that experiments by Eriksson, Klebahn, 

 Stakman, and many others have amply demonstrated the existence 

 of specialized strains, races, and forms among cereal rusts. For 

 example, Fiiccinia graminis consists of: 



{a) P. gravtinis tritici on Triticiivi viilgare. 



(b) P. graminis secalis on Secale cereale and other hosts. 



(c) P. graminis avenae on Avena sativa and other hosts. 



(d) P. graminis airae on Air a caespitosa. 



(e) P. gravjinis poae on Poa compressa and other species. 



(f ) P. grajiiinis agrostidis on Agrostis caninis and other species. 



In experiments extending over a long period, Stakman et al. 

 (1934) have shown that barberry is essential for the production 

 and perpetuation of varieties and forms of Piiccinia graminis. Of 

 675 aecial collections 34.2^0 were of the tritici variety, 63.7% of 

 the secalis variety, and 2.1% of the avenae variety. Their evi- 

 dence led them to conclude that the origin and persistence of the 

 secalis, poae, and agrostidis varieties are dependent upon the pres- 

 ence of barberries. They found the physiologic forms of P. 

 graminis tritici to belong within 26 distinct form groups. In 8000 

 collections of urediniospores a new form was encountered about 

 once in 100 collections, whereas in 71 collections made near 

 rusted barberries 19 belonged among the 26 recognized forms, 

 and 1 had never before been encountered within the United 

 States. 



