46 A STUDY IN CEREAL RUSTS 



unsuccessful attempts to form pustules are often noticed. The 

 sequence of events in Arnautka 288 and Kubanka 2094 is quite similar 

 to that in Khapli. These are semi-immune forms in which the contest 

 between host and parasite is somewhat prolonged. Fatalities occur on 

 both sides but not in sufficient number to render infection absolutely 

 unsuccessful. It is, therefore, essential to note the sequence in such 

 extreme cases of almost total immunity as are furnished by wheat 

 when inoculated with Puccinia graminis avenae. 



MINNESOTA NO. 163 INOCULATED WITH PUCCINIA GRAMINIS AVENAE 



It will be recalled that very rarely indeed does successful infection 

 follow inoculation of wheat with Puccinia graminis avenae. Long 

 germ tubes are sent out by spores germinating on the surface of the 

 leaf. These follow the epiderm, dipping into depressions in an entirely 

 normal manner. Appressoria are found, a small neck grows through 

 the stomatal slit and the substomatal vesicle is formed. This vesicle 

 sometimes sends out only very small knoblike branches which soon 

 die, or branches may be sent out and very few hausteria produced. 



The vesicle often sends out many short, knoblike branches, 

 appearing almost like elongated amoebae. These do not appear 

 vigorous after a few days. The epiderm cells in the immediate vicinity 

 appeared to be killed. If definite infection threads are sent out they 

 never grow very long but kill two or three host cells and then stop 

 growing. 



The difference between conditions in such a case as this and 

 such a one as Khapli is apparently in degree only. Whereas in Khapli 

 the fungus might develop to a certain extent, thus involving fairly 

 large leaf areas, in such an extreme case of immunity as is presented 

 by wheat to oat rust only a few host cells are involved and the contest 

 between host and parasite is short and decisive, only a very few host 

 cells being killed; and the hyphae seldom develop sufficiently to give 

 any external evidence that the germ tube has even entered. 



As to the fundamental causes for these facts, only speculation is 

 possible. It would seem reasonable to suppose either that there was 

 a lack of an attracting substance or the presence of a deleterious sub- 

 stance. Massee's chemotaxis theory has already been mentioned. 

 Chemotaxis or its absence would hardly explain the phenomena men- 

 tioned, since the fungus succeeds in effecting an entrance into even the 

 most immune forms. The evidence would rather seem to favor the 

 view that the whole problem is one of toxins in host or parasite or, 

 very probably, in both. In some cases the host is apparently hyper- 

 susceptible, while further study may prove that there is in other cases 

 a real resistance. Careful biochemical investigation alone can settle 

 this question definitely. 



