258 PHYSIOLOGIC SPECIALIZATION AND VARIATION 



f ection if inoculated onto other grass hosts, such as rye, oats, timo- 

 thv, and blue grass, whereas under the same environmental condi- 

 tions wheat readily became infected. Similarly Fuccinia graminis 

 from any of the other grasses produced infection on the host 

 species from which the inoculum was taken, but reciprocal inocu- 

 lations always failed to cause infection. Further work of the same 

 nature by Eriksson (1894) led to the division of Puccinia graminis 

 into the following groups, which he called "formae speciales": 

 Fuccinia graminis tritici, P. graminis secalis, P. graminis avenae, 

 P. graminis phlei-pratensis, P. graminis agrostidis, and P. graminis 

 poae. He also showed that subdivisions can similarly be made of 

 P. ghimavum, P. dispersa, and P. coronata. He recognized five 

 specialized forms, tritici, secalis, elymi, agropyri, and hordei, of 

 P. ghimavum. Four specialized forms, secalis, agropyri, bromi and 

 tritici, comprise P. dispersa; and P. coronata consists of six, avenae, 

 alopecnri, festucae, lolii, calamagrostis, and vielicae. To these 

 groupings within the species the terms biologic forms, biologic 

 races, physiologic forms, biologic species, physiologic species, 

 physiologic races, parasitic strains, sister species, and specialized 

 varieties have been applied. They are now generally regarded 

 as varieties, and many workers designate them as of varietal rank. 

 Their pathogenic behavior thus serves as the basis for the varietal 

 separations. Within the past 25 years it has been found that many 

 parasitic strains or biotypes comprise a given variety and that 

 some of these strains can be isolated by their pathological effects 

 on appropriate suscept species, and others by cultural character- 

 istics. It is to these strain groupings that the term physiologic 

 specialization is properly applied. 



Some mycologists maintain that it is impossible to establish 

 varieties among pathogenic fungi, as in Fuccinia graminis, on the 

 basis of morphologic differences. If this be true, there is little 

 justification for the use of varietal names. Minute yet recogniz- 

 able differences are indicated by others to exist, and they there- 

 fore find it convenient to employ varietal names. Without the 

 threadbare problem of what constitutes a variety or physiologic 

 species being raised again, it is clearly established by students 

 of rusts and smuts that secondary groupings within the variety 

 may be made on the basis of pathogenicity on selected suscepts. 

 These secondary groupings are called physiologic strains. They 



