A STUDY IN CEREAL RUSTS 

 PHYSIOLOGICAL RACES 



PART I. BIOLOGIC FORMS 



HISTORICAL 



Cereal rusts were known as destructive plant pests by the ancients. 

 Observations were made on the effect of weather and location on their 

 prevalence. The comparative susceptibility of the various cereals 

 received some attention. Theophrastus speaks of the varying suscepti- 

 bility of different cereals, as does Pliny, who says that barley is less 

 likely to rust than are other grains. In modern times many observa- 

 tions have been made, but many of these were only incidental. 



Dietel (1887) calls attention to a certain amount of morphological 

 and physiological variation in rust fungi, but does not definitely estab- 

 lish the fact that there are distinct biologic forms. Previous to his 

 time Puccinia graminis Pers. was considered a single species found 

 on cereals and various grasses. However, in 1894 Eriksson (1894-1) 

 showed that, although the morphology of the fungus on the different 

 cereal hosts varied but slightly, there was a distinct specialization in 

 parasitism. He therefore divided the different species of rusts into 

 subdivisions which he termed "formae speciales." Puccinia graminis, 

 the only species extensively used in the present investigation, he di- 

 vided into five formae speciales, two of which he mentioned as being 

 sharply demarcated and the remainder as being probably also distinct. 



All of these forms of Puccinia graminis were capable of produc- 

 ing aecidia on various species of Berberis. It therefore occurred to 

 Eriksson that perhaps his formae speciales would be equalized when 

 grown upon the alternate host. This, however, he found not to be the 

 case. He concluded that the forms were physiologically distinct even 

 when grown upon Berberis. In fact he was led to believe that they 

 became more firmly fixed after a period on the barberry. 



This discovery stimulated much research concerning the physiolog- 

 ical relationships of the Uredineae. In the United States Hitchcock 

 and Carleton (1894) made observations at about the same time that 



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