DIFFERENCES IN ARTIFICIAL CULTURE 



263 



The existence of adaptation and "bridging" among pathogens 

 remains questionable in the light of these experiments. That the 

 pathogenicity of specialized races is hereditary and therefore con- 

 stant, on the other hand, has volumes of evidence in its support. 

 Those who have studied the rusts over a period of years, as have 



H. hordeaceus 



H. secalinus 



H. arduennensis 



H. interruptus 



H. commutatus 



H. racemosus 



Fig. 46. Reciprocal inoculation of species of Hordeum with conidia of 

 powdery mildew. Solid lines show successful transfer with resultant in- 

 fection in direction indicated bv arrow; broken lines, failure of infection 

 after transfer of conidia. The numerator indicates the number of successful 

 trials; the denominator, the number of attempts made to secure infection. 

 The species hordeaceus is a "bridging species" for Erysiphe gram'inis. 



(After Salmon.) 



Stakman and his associates, or the smuts, as have Reed and his 

 associates, are able to isolate the same physiologic forms year 

 after year. These identical forms may range widely in one area 

 or country or may even be found in different continents. 



Differences in artificial culture. Macroscopic or micro- 

 scopic differences between strains isolated from monosporous cul- 

 tures have been reported for numerous species of fungi. From 

 our knowledge of genetics these strain differences may have arisen 

 either through hybridization or through mutation. 



From the voluminous literature on plus and minus strains within 



