400 BROOKLYN BOTANIC GARDEN MEMOIRS 



The notion of bridging hosts, of course, impHes that the fungus 

 undergoes a physiological change in consequence of its new habitat 

 and thus becomes able to attack other hosts. The change is certainly 

 closely associated with variation in virulence so well known in the 

 case of pathogenic bacteria. It is quite likely that fungous parasites 

 vary in virulence or can be made to do so by suitable experimental 

 methods. At present, however, we have no good evidence that this 

 has been done in any particular case. The results of Evans (44), 

 referred to above, point in this direction. In this case, however, the 

 data are not at all extensive. The facts might be explained by an 

 increase in susceptibility of both resistant and susceptible parent in 

 response to a change in the environment; or external factors may 

 have favored a more successful invasion on the part of the rust parasite. 

 Passing reference may be made to the work of Salmon (129, 130), 

 Ward (173), Stakman (143) and Spinks (141) which clearly indicates 

 that a plant may be rendered more or less susceptible to fungous 

 invasion by means of certain agencies. The work of these investi- 

 gators shows that mineral starvation, excess of nutrients, mechanical 

 injuries, anaesthetics, etc., modify the relations of a plant to fungous 

 invasion . 



It has been pointed out by Eriksson (35), Ward (172, 174), Vavilov 

 (164, 165) and others that a specialized race tends to occur on more or 

 less closely related hosts. There are, however, great differences among 

 the specialized races in this respect. As pointed out above, the host 

 range of these races may be narrow or wide. Within a single species 

 of parasite we may have a race occurring on many hosts belonging to 

 different genera and another race restricted to a single genus or even 

 certain species of a genus. Piiccinia graminis, as well as other fungi, 

 includes races of such wide differences in host range. 



Attempts have been made to utilize the infective capacity of a 

 parasite to determine the genetic relationship of hosts. Eriksson (35) 

 applied this test in determining the possible relation of a rye-wheat 

 hybrid to the two parents. Ward (171, 172) reports a fairly close 

 correspondence between the hosts of the more or less well-defined 

 races of Puccinia dispersa bromi and the grouping of the bromes on 

 other grounds. Vavilov (154, 155) has used Puccinia dispersa tritici 

 and Erysiphe graminis tritici as a test in determining the relationship 

 of types and varieties of Triticwn. He also used Puccinia graminis 

 avenae and P. coronata avenae as a similar test in connection with species 

 and varieties of Avena. It is interesting to note that the parasites on 

 wheat gave practically the same results and these are both quite 

 narrowly specialized races. The rusts on oats, however, did not give 

 corresponding results, Puccinia graminis avenae infecting a wider 



