PATHOGENICITY TESTS 



261 



Although the list in Table 26 is by no means complete, it indi- 

 cates that physiologic specialization occurs among all the major 

 groups of fungi. A survey of accounts from which this list was 

 compiled shows that in the recognition of physiologic forms four 

 criteria were employed: (a) pathogenicity on special hosts, (b) 

 differences in artificial culture, (c) minor morphological differ- 



40 



30 



bo 



E= 20 



Hi 



10 







14 



18 



22 26 30 

 Length of spores (microns) 



34 



38 



Fig. 45. Variation in length of spores of six strains of Pestalozzia guepini, 

 plotted as length of spores in microns against percentage of the total 



number measured. (After LaRue.) 



ences, and (d) physico-chemical reactions. Each will be given 

 further consideration. 



Pathogenicity tests. Concerning the pathogenic potentialities 

 of disease-producing fungi, two diametrically opposed theories 

 have been advanced. One is that disease-producing potentialities 

 are inherent in the fungus itself and therefore are hereditary. 

 The other is that the pathogens become adapted, modified, or 

 "educated" under the influence of the host or of other environ- 

 mental factors. According to the first view, the physiologic 

 forms are true-breeding entities that maintain a uniform potential- 

 ity to produce disease throughout many generations or over a 

 long period of years. The weight of evidence in recent years 

 favors this viewpoint, since physiologic characteristics appear to 



