160 T O R R E Y A 



Poae. Another step was taken in 1917, when Stakman and Piemeisel found that 

 P. graminis tritici consisted of at least more than one specialized race or physio- 

 logic form. By 1922, 37 specialized races of this pathogen were known and by 

 1934 not less than 127 had been isolated, and now the number is about 160. 

 Similar specialization has been found in other groups of grass rusts. In crown 

 rust of oats Murphy (1933) listed 33 races and Johnston et al (1942) brought 

 together the data for Puccinia ruhigo-vera tritici, recording 129 races known in 

 A-arious parts of the world. Most rusts which occur on several species of grasses, 

 particularly if they belong to different genera, show the phenomenon of special- 

 ization. It is interesting, however, that Puccinia suhnitens does not, the spores 

 from the uredial and telial host being able to infect aecial hosts belonging to 15 

 genera, distributed among 6 different families. 



Specialized races of the powdery mildews were first recorded by Marchal 

 (1902) when seven were dift'erentiated on the basis of cultural experiments — 

 Avenae, Agropyrac, Broiui, Hordei, Poae, Secalis, and Tritici, all being limited 

 to one or more species of a single genus. Salmon (1903j and Reed (1906- 

 1916) extended the evidence for specialization within this mildew. A further 

 step was taken by ]\Iains and Dietz (1930) when they showed that ErysipJie 

 graminis hordei consisted of at least 5 distinct races, and Mains (1933) found 

 2 races of E. graminis tritici. 



The first evidence of specialization in the smuts was recorded by Zillig 

 (1921) in Ustilago z'iolacea. Faris (1924) demonstrated the occurrence of 5 

 phvsiologic races in the covered smut of barley, U. hordei, and Reed (1924) 

 demonstrated races in both loose and covered smuts of oats. At the present time 

 30 speciaHzed races of loose smut and 14 of covered smut are known. Faris 

 ( 1924) demonstrated specialization in the bunt of wheat, his data being ex- 

 tended bv Reed ( 1927. 1928 ) when 5 races of Tilletia levis and 6 of T. tritici 

 were dift'erentiated. Rodenhiser and Holton (1937) listed 8 physiologic races 

 of T. levis and 11 of T. tritici. Such specialization has also been found in 

 SphacelotJieca sorghi, Sorosporiinn reilianum, Ustilago tritici, and U. seae. 



Physiologic specialization is an essentially universal phenomenon among 

 the pathogenic fungi. In any case where a morphological species of a fungus 

 occurs on several hosts, it is almost certain that strains or races exist which 

 are limited in their capacity for producing infection. 



Ward (1903). in his study of the brome rust. Puccinia dispersa. raised the 

 question whether ''bridging hosts'" existed, publishing data which he regarded 

 as evidence that a particular race of brome rust might be grown on a specific 

 host and then be capable of infecting other brome grasses which originally it 

 was not able to do. Salmon (1904) published similar data for the powdery 

 mildew on the brome grasses. For many years no clear-cut confirmation of 

 these conclusions was available. The general idea, however, was held in con- 



